Difficulty determining correct o-rings for older Mares pneumatics

[spoolin01]

I dug into an old Frontiersman that has been sitting in my garage for years, but have been unsuccessful in determing which o-rings to use for the tank, which I gather are metric. The existing o-rings have hardened to plastic, so I don't trust their current dimensions. I generally find that o-ring dimensions can be fairly tolerant for many applications and still work, but for the tank I've tried both the 028 and 124 standard o-rings, which are clearly different thickness than OEM, and the 028 doesn't seal, and I can't force the tank over the 124s. I've used standard o-rings in the past on other guns, but that's not working here. It appears there are no other suitable prospects in the standard sizing scheme.

Frustrated at the failure of my usual DIY approach, I went to the local Mares dealer, who's got some great old pneumo guns on his wall (Bamboo Reef in San Francisco), and after consultation with Mares that this kit would service the Frontiersman, he gave me the 163979 kit (Mares part-number labeled but in an unsealed zip-lock). The barrel and piston o-rings, and piston skirt in the kit appear to be of the same general size as those that came out of the gun, but the tank o-rings are only ~31mm ID. I've got some original blister-packed Cyrano/Spark 164290 kits as well, and the tank o-rings in the two kits are the same size. Confusingly, I've got an undated Cyrano service manual that shows the 163979 kit as being for the Cyrano, while it's clear that at least 3 of the o-rings in the 163979 kit are not present in the current 164290 kits including the piston o-ring, and the seal in the Cyrano/Spark kits is clearly smaller than that in the 163979, as it seems you would expect!! So that has me confused - I suppose the service manual could have listed the wrong kit... or the unsealed kit I got from the dealer could have been altered - it had a dealer label on it that just said 'various Mares'. However, the partial 117 page circa 2001 "Mares Dealer manual" that circulates widely on the web which has part numbers only for the Sten gun - which I read here is similar to the older guns - shows the 163979 as being for the Sten. So maybe I did get a proper 163979 kit, or at least a partially proper one.

The handle nose grooves, and the nose cone, are 1.375" or 35mm diameter. The hardened original o-rings measure about that ID. A try-fit of the 31mm o-ring doesn't require too much force to stretch over the nose cone, but that represents about 11-12% linear stretch, which made me wonder if these are right for the gun. The thickness is intermediate between the 028 and 124 so I'm guessing these will work, but was curious if they were indeed OEM spec for this gun.

A couple of questions might help clear things for me:

Are the tanks of 11mm and 13mm guns the same diameter?

Does anyone know the dimensional specs of the tank o-rings on those older guns? The kit o-rings appear to be about M2.4 x 31.6 (matching measurements to the closest metric size I found on the internet). Whichever turns out to be correct or suitable, I'd like to order some of the tank and barrel o-rings from an o-ring house...

 

[spoolin01]

Some progress perhaps in returning the gun to functioning, if no greater clarity about OEM configuration.

I put the 31mm tank o-rings in and proceeded to try screwing the head on. There is a groove in the barrel an inch or so back from the end threads, with a ~ 17mm diameter. There were two o-rings in the kit with 16mm IDs, one about 3mm thick, the other about 1.8mm thick. The o-ring in place when I disassembled the gun looked very much as thick as the 3mm o-ring, although deformed. It was not sitting squarely in the groove when I first saw it, but that might well be because although the groove sits below the end-plane of the nose cone, the internal diameter of the nose cone at it's smallest, will not fit over most o-rings sitting in the groove, and they get dislodged pulling the nose cone off. So with the nose cone in place, you have to stuff the o-ring about an inch down the barrel inside the nose cone to set it into the groove. Comparing the OD of the 3mm x 16mm o-ring to the bore of the head, it's clear that either it's the wrong o-ring, or it's not mean to fill the space between the barrel groove and the head's inside bore - there's just too much material there to stuff into that space, and since only the shallow groove positions the o-ring while screwing down the head, the 3mm o-ring gets wiped out of the groove in the process. So, thinking a previous owner may have put in the wrong o-rings, and not seeing another 16mm o-ring in the old gun parts pile or in the 1983 Mares exploded diagram I found on the web, I figured maybe the thinner one was the barrel o-ring for this model, so I tried the M1.8x16, which is a lightly snug fit inside the head bore, while sitting in the groove. However, the gun leaked badly from the muzzle when I pumped it with the muzzle in a bucket of water, and 40 strokes produced only a brief whisper of release when I depressed the fill-valve ball. Most of the bubbles came from the head-nose cone joint, a few appeared to come from the nose cone-tank joint. Going back to the thesis that the 3mm o-ring was the right one, I figured that could only mean it was meant to be sandwiched between the rear face of the head, which is slightly chamfered on the inner circumference, and the internal bulkhead of the nose cone, which is of the correct diameter and positioning to sandwich the o-ring. When screwed down, the head doesn't quite close the gap with the nose cone using moderate hand-tightening. A little more might do it. That still leaves the purpose of the groove a mystery - I have doubts that this sandwiching approach is correct.

The good news is, the gun appears to hold some pressure after 40 strokes. It still bubbles slowly but now from the ports, so I guess the piston is leaking. I didn't replace the skirt seal since it looked OK and supple still.

Do you think that leak could be because I haven't put in enough pressure to force a good seal around the skirt? The barrel bore looked completely smooth, and I wiped it clean. I put 25cc of 15W moly fork oil in the gun but then lost or poured much of that out during the re-checking of o-rings, etc. I don't want to button the gun down and pump it fully until I've got some 5W or 10W to add. I'm going to try to work up a fitting so I can pressurize the gun at least part way from the LP side of a scuba regulator.

One of the members here kindly agreed to forward a Sten service manual, so maybe that will hold some answers or clues to how this thing is built.

Also, I don't see a way to take the head piece apart to get at the shock absorber pieces - it doesn't look like the 1983 version, no flats or knurls, and the nose cone and head look different. There are some flutes where the ports are that you could get a pin spanner on, but if there's a seam there, or anywhere else, it's an amazingly fine one. It also doesn't appear those parts can be driven out the back end of the head piece, though I haven't tried that with any force. I don't know that they require any servicing or inspection, I just like to take stuff apart. I'll take pics when I pull it apart again to add oil.

 

[popgun pete]

The "O" rings for the tanks are the same for all Mares rear handle models, but are of a special ring size. That particular "O" ring, as you have found out, needs a stretch to fit as its ID is smaller than the diameter of the groove it sits in on the moulded components. I had a packet of those Mares tank rings, so I quit experimenting with stock sizes that never seemed to fit, but the key dimension to get right is the "rubber width" of the ring, or its "section width". I did have success with the "Titan" tank "O" rings as I had no choice but to replace them with stock sizes as the counter-bored end, thicker walled tanks of the "Titan" use a different size "O" ring to the "Sten" models. You can slightly stretch the diameter of an "O" ring, but if the section width is wrong it either will not seal or it will be impossible to push on the tank tube or whatever component slides over the top of it. Some "O "rings used in the Mares guns are standard stock sizes, such as the piston "O" ring in the "Sten". The "Cyrano" inner barrel tube is a smaller OD than the one in the "Sten", so "O" rings that fit into the machined barrel grooves are not the same for the two guns. Changes to the interior of the "Sten" nose cone over time has meant that fatter section width "O" rings that previously worked on the front end of the inner barrel tube do not work on later models, they have to be of a smaller section width. Given the time spent putting a gun back together after cleaning everything off I recommend that Mares "O" ring kits are used, if they are available, or you end up performing the work twice if you find the stock size replacement rings are not sealing. Sometimes it can be very close to doing so, but you are forced to dismantle the gun again.

Some older Mares muzzles (but not the oldest) are one-piece construction, everything pushes in from the rear end, unlike others which have a screw in nose piece, although I always work from the inner rear end on those muzzles as well. The one-piece muzzles are seen on Mares "Reef" models, but I have also found them on "Sten" guns. The "Reef" is a continuation of the original "Sten" shape when the "Sten 87" or "Competition Line" gun came out. The line release was moved to the side of the finger guard on both guns in order to avoid the line release finger acting as a second trigger if you were stupid enough to squeeze the wrapped shooting line onto the reservoir tube of a cocked gun with the safety "off". The "Competition Line" model is now produced as the "Jet" with a few modifications, resurrecting the name of the first Mares pneumatic speargun. I cannot comment on the "Sten 2001" as I have yet to pull one of those apart.

In the "on-line" version of the Mares Manual that you mentioned there is an "O" ring type reference page showing the sizes of the various rings used in their speargun range. I worked my way through it and have found that it shows most of the sizes used in the past, but by my reckoning there are four sizes missing which are from the "Mirage".

I will show the "O" ring application list in another post, I just have to find my notes. Some "O" rings have been assigned different identification numbers over the years, but are still the same size as before. A few rings have been made of harder rubber in the later versions, particularly for the "Mirage".

 

[popgun pete]

Here are the gun applications for the various "O" rings used in Mares spearguns since the first "Sten" appeared. The order of the rings is that shown on the Mares "O" ring type reference page in the cut down, "on-line" version of the Mares manual. I believe it was originally an instructional handout for training purposes, so is not the full version. The individual "exploded diagram" part numbers for each ring are shown in brackets. "Sten" is the first version of the gun until the rear grip shape and muzzle changed. "Sten 87" is the "Competition Line" model, "Sten 2001" is the current model with the "Cyrano" style rear handle. "Reef" is a continuation of the original "Sten" shape, but with modifications to reduce its production cost (some parts used less plastic, some parts were either simplified or omitted).

The part reference numbers now all start with a "46110", those that do not are from earlier versions and have no counterpart today, although that does not necessarily mean that they are no longer available. Last three number actually identify the component and in some cases the numbers have carried over from the earlier code used for the item.

OR 2003 46110242 trigger seal on "Apnea" system Cyrano (49) Sten 2001 (49)

OR R/1 46110201 trigger seal on Sten (31), Reef (46), Mirage (28); inlet ball valve seal on Sten 87 (46), Sten 2001 (46), Reef (46)
on original Sten reference number was 110201 (31), seal mounted on a moving piston instead of surrounding a simple pin


OR 2007 46110213 trigger seal Sten 87 (77), seal sat in a removable plastic stirrup with two prongs holding it in the handle

OR 2015 (was OR 102) 46110102 power regulator shaft seal and power regulator piston seal
on original Sten reference number was 110102 (27)


OR 106 46110106 safety lever friction element Cyrano (60), Sten 2001 (60), pumping barrel rings Mirage (46)

OR 2031 46110107 piston ring 11 mm barrel Cyrano, Spark, Stealth

OR 2062 46110220 barrel to pre-chamber bulkhead for 13 mm barrel Reef (71), Sten 87 (71), Sten 2001 (71)
on original Sten reference number was 110121 (17)



OR 122 BIS 46110206 barrel to nose cone seal 13 mm barrel Sten (9), Reef (22), Sten 87 (22), Sten 2001 (22)
on original Sten reference number was 110206 (9)


OR Special 12 BIS 110012 barrel to nose cone and bulkhead Mirage (43)

OR 2043 110215 replaces OR 106 in later Mirage at front of pumping barrel

OR 2037 (was OR 110) 46110110 piston ring 13 mm barrel Reef (18), Mirage (9), Sten 87 (18), Sten 2001 (18)
on original Sten piston there is no "O" ring, just one piece "back to back" double cone seal

OR 3062 46110228 barrel to nose cone and inner nose cone support Cyrano (22)

OR 3056 46110227 barrel to pre-chamber bulkhead Cyrano (71)

OR 2106 46110245 nose cone inner support to nose cone Cyrano (29)

OR Special 46110208 main body or tank ring all guns Reef (20), Mirage (36), Sten 87 (20), Sten 2001 (20), Cyrano (20)
on original Sten reference number was 110208, each ring had its own diagram number (6,19,49)

OR 101 1101012 ball valve seal Mirage (15), OR 2012 in later Mirage both shore hardness 75
on original Sten reference number was 110200 (43) for stem type valve stem


OR 105 1101057 inlet valve protective cap seal, bayonet locking cap Mirage (12) and all early Sten models
on original Sten reference number was 110205 (47), later caps screw in and have no seal

 

[popgun pete]

The "O" ring at the muzzle sits on the outer surface of the inner barrel, i.e. on the OD, it does not sit in a groove. The groove is there for a circlip or in later models a split plastic ring that fits into the groove and provides the backing for a round plastic collar that slides up against it from the front end. That collar in turn backs the muzzle "O" ring and holds it against the nose cone interior's front wall so that you achieve a seal where the inner barrel passes through it. On a "Mirage", where the inner barrel is mounted off-centre and higher up in the gun, the muzzle "O" ring does sit in a groove as the ends of the gun tend to tip slightly upwards when under pressure (like a banana), so to ensure a seal the muzzle "O" ring is more positively located in a groove. To make doubly sure of this sealing at the nose cone the early "Mirage" had two grooves and two "O" rings in that location.

On older "Sten" guns I have pushed the muzzle "O" ring onto the inner barrel to a position just past the muzzle screw threads and then allowed the nose cone to push it further along the barrel during re-assembly. Pressurizing the gun drives the "O" ring forwards and seals it without the collar, in fact I have left the collar off when fixing some older guns. However if a small gap is present between the "O" ring and nose cone interior's front wall then the "O" ring will not seal, so that is why Mares added the plastic collar to ensure that it was pressed up behind the nose cone's front wall before pumping starts. On early guns the plastic collar is simply a tight fit, it does not index at any fixed fore-aft position on the inner barrel. Tiny abrasive articles trapped under the collar can scratch the inner barrel's outer surface; I polished a barrel a couple of times and still had a muzzle leak. Then I noticed pushing the collar on added a new scratch, so I left it off, it must have had a bit of abrasive stuck in the plastic which was too small to see.

 

[spoolin01]

The "O" ring at the muzzle sits on the outer surface of the inner barrel, i.e. on the OD, it does not sit in a groove. The groove is there for a circlip or in later models a split plastic ring that fits into the groove and provides the backing for a round plastic collar that slides up against it from the front end. That collar in turn backs the muzzle "O" ring and holds it against the nose cone interior's front wall so that you achieve a seal where the inner barrel passes through it.
<snip>
On older "Sten" guns I have pushed the muzzle "O" ring onto the inner barrel to a position just past the muzzle screw threads and then allowed the nose cone to push it further along the barrel during re-assembly. Pressurizing the gun drives the "O" ring forwards and seals it without the collar, in fact I have left the collar off when fixing some older guns. However if a small gap is present between the "O" ring and nose cone interior's front wall then the "O" ring will not seal, so that is why Mares added the plastic collar to ensure that it was pressed up behind the nose cone's front wall before pumping starts. On early guns the plastic collar is simply a tight fit, it does not index at any fixed fore-aft position on the inner barrel. Tiny abrasive articles trapped under the collar can scratch the inner barrel's outer surface; I polished a barrel a couple of times and still had a muzzle leak. Then I noticed pushing the collar on added a new scratch, so I left it off, it must have had a bit of abrasive stuck in the plastic which was too small to see.

Eureka!! This is the part I wasn't getting. The groove just behind the threads had me convinced the barrel o-ring had to go there, to the point of tunnel vision. Yet the exploded diagram of the early Mares guns clearly showed it BEHIND the nose cone bulkhead, and a little thinking should have made it obvious that placing it in that groove would mean the seam between the head piece and the nose cone would lie between the two seals, and of course it would leak just like it did. By the way, crushing it behind the head piece did seal it (so it was just on the forward side of the nose cone bulkhead, not the rear side where it should be), but the fit just didn't look right.

I located some 5W fork oil, cleaned all the parts again, and re-assembled the gun this morning. Voila, no leaks when I pressurize and submerge the gun!! I ended up using the tank, barrel, and piston o-rings from the Mares kit, the rest were just picked from the standard US sizes of o-rings. There is a standard-sized o-ring that closely matches the OEM piston o-ring, though I didn't use it here.

I took some pictures that I'll post in case anyone else can learn from this.

Thank you pete, for your help.

 

[spoolin01]

Here are some pics of the parts. Now that it's back together it all seems obvious, but it gave me some trouble at the time. Maybe others will find this helpful. The barrel o-ring does NOT sit in the groove on the barrel!

 

[spoolin01]

Here is the head piece from this particular gun - it appears to be one piece. Looking down the bore from the back, it's not obvious that the shock absorber parts could be forced out the back end. They look larger than the bore, and possibly glued in.

 

[spoolin01]

Here is the exploded diagram for 1983 Mares pneumo guns. This looks close to the parts in my Frontiersman, but not exact (the nylon bushing behind the nose cone is different, for instance). The diagram of the Sten 2001 in the online "Mares dealer manual" is quite a bit different.

 

[spoolin01]

Here is a comparison of the Cyrano/Spark service kit 164290 with the kit I bought from the local dive shop, 163979. When I took the pic, I had already used the 3 tank o-rings, the barrel o-ring, and the pistion o-ring from the 163979 kit. The Cyrano kit has the needed number of suitable tank o-rings, and apparently two of the thick barrel o-rings (only one is needed for the Frontiersman) that will work on the 13mm guns, but no piston o-ring for the 13mm piston. The tank o-rings are really the only ones that can't be readily substituted with standard US-sized o-rings, that I can see.

 

[popgun pete]

Well it is very interesting to finally see this version of the "Sten" series with this particular muzzle and nose cone combination. I remember seeing them in full page adverts in "Skin Diver" magazine in 1981, in fact I am looking at one right now (May 1981 issue, p.17). The Mares spearguns were then being distributed in the USA by SeaQuest. I was always curious as to why this "buttressed muzzle" was adopted when later on Mares reverted back to the usual "socket nose" type of nose cone and the basically cylindrical, machined muzzle that fitted inside it. I never saw this "buttressed muzzle" model sold here, it may have only been produced for a relatively short period and by the time our national distributor brought in a new batch of Mares guns that model was already replaced as the previous stock of "Sten" guns took a long time to sell. At times the same gun could spend a year on the shop wall as no one was interested in buying it!

The "head" (testata) is the muzzle and it looks like it is a machined casting as otherwise I cannot see how the buttress shapes were generated that flank the four relief port holes. If that muzzle is an all metal housing then it will be heavy, but maybe it is lighter than it looks. The white barrel bushing is much larger in diameter than they were before when it was essentially a small diameter plastic tube. The latest version of this bush is a split plastic ring with a ridge in the centre on the inner surface that performs the function of both circlip and bush, or what I referred to earlier as a collar. The inner ridge sits in the barrel groove and the front of the bush backs the muzzle "O" ring. On "Cyrano" guns there are two of them. You need small nose circlip pliers to remove them as otherwise they can snap if you spread them too far to clear the groove.

Thanks for the nice photos as I have never seen these front end parts before, in fact I am going to have to think about why they made the muzzle this way, there must have been a reason for doing so. Whatever it was by 1983 Mares went back to the muzzles that we usually see on their guns. Sometimes cast alloy components cannot take high energy impacts over a sustained period, but until we hear from someone else this is going to be a bit of a mystery.

 

[spoolin01]

I'm amazed that this gun may actually be older than those in the 1983 diagram! The gun really doesn't reveal it's age in any respect, except that it's in relatively good condition if it is that old.

The muzzle is just plastic, like the nose cone. There appears to be a fine seam where the flutes meet the outer circumference, but it's very fine, and in looking between the flutes, I don't see confirmation that this is actually an assembly joint. I had thought that the flukes might have been for the purpose of disassembly using a pin spanner or the like, but I believe it's a one-piece muzzle. The barrel threads into a metal cylinder located mid-way up the muzzle bore, that backs additional components, the shock buffer I suppose.

I'll try to take better pics of it after the weekend.

 

[popgun pete]

I zoomed your photos to have a better look at everything and can now see how it goes together. On a "Sten" from both before and after this the barrel "O" ring sits against the inner wall of the nose cone as the hole through the nose cone is only very slightly larger than the OD of the inner barrel tube, so that is where the inner pressure seal takes place. As usual the outer tank to nose cone seal takes place at the flange of the nose cone that projects into the tank tube. The nose cone shown here is rather truncated and has a large diameter hole right through it with probably an inner internal step hidden at the front end, so the barrel "O" ring must seal on the rear of the muzzle itself, not the nose cone, which is very different. The nose cone sits on the muzzle's reduced diameter rear extension which passes right through it, so the annular step to the largest diameter section of the muzzle must seal on the inner front face step of the nose cone without needing a sealing ring (unless there is one there which we have not seen). That makes for a pressure seal similar to the one behind the inlet valve body, there is no seal there either between the outer flange of the valve body and the moulded recess in the rear of the handgrip moulding. Looking more closely at the muzzle you will have to tell me how many relief ports there are as one appears to have no hole between the buttresses; if there are six buttresses then maybe there are only three holes with the intervening spaces blank.


Checking the old advert it looks like only the "Shortie", "Frontiersman" and Californian" received this buttressed muzzle, the other models appear to have the usual style front end.

 

[popgun pete]

The muzzle is just plastic, like the nose cone. There appears to be a fine seam where the flutes meet the outer circumference, but it's very fine, and in looking between the flutes, I don't see confirmation that this is actually an assembly joint. I had thought that the flukes might have been for the purpose of disassembly using a pin spanner or the like, but I believe it's a one-piece muzzle. The barrel threads into a metal cylinder located mid-way up the muzzle bore, that backs additional components, the shock buffer I suppose.

I'll try to take better pics of it after the weekend.

Now that you have told us that the muzzle is moulded plastic I can guess why it was not persevered with; plastic is too weak for the job even if the attachment screw threads are reinforced with a metal insert moulded into the muzzle body. Using the gun repeatedly at higher pressure could bust the muzzle at the ports; now we know why they are buttressed and some appear to be blank! Eliminating metal and thereby reducing machining costs will have been the motivation for the change to plastic, I can see why they tried it, but subsequent history shows that Mares abandoned the idea. Still very interesting to see one and I look forward to some more photos looking in from the ends if you could take some. If you stay within the recommended pressures then the gun should work OK, sometimes one is tempted to lean on the pump handle and gain some extra range, but not with that muzzle.

 

[spoolin01]

I zoomed your photos to have a better look at everything and can now see how it goes together. On a "Sten" from both before and after this the barrel "O" ring sits against the inner wall of the nose cone as the hole through the nose cone is only very slightly larger than the OD of the inner barrel tube, so that is where the inner pressure seal takes place. As usual the outer tank to nose cone seal takes place at the flange of the nose cone that projects into the tank tube. The nose cone shown here is rather truncated and has a large diameter hole right through it with probably an inner internal step hidden at the front end, so the barrel "O" ring must seal on the rear of the muzzle itself, not the nose cone, which is very different. The nose cone sits on the muzzle's reduced diameter rear extension which passes right through it, so the annular step to the largest diameter section of the muzzle must seal on the inner front face step of the nose cone without needing a sealing ring (unless there is one there which we have not seen). That makes for a pressure seal similar to the one behind the inlet valve body, there is no seal there either between the outer flange of the valve body and the moulded recess in the rear of the handgrip moulding. Looking more closely at the muzzle you will have to tell me how many relief ports there are as one appears to have no hole between the buttresses; if there are six buttresses then maybe there are only three holes with the intervening spaces blank.


Checking the old advert it looks like only the "Shortie", "Frontiersman" and Californian" received this buttressed muzzle, the other models appear to have the usual style front end.

Very close in your analysis!! However, there is what I called a 'bulkhead' inside the nose cone. The back end of the head piece/muzzle sits close or against the forward side of the bulkhead, the barrel o-ring seats against the rear side of that bulkhead. In this fashion, all the pressure sealing occurs at the nose cone, and none is needed for the muzzle piece. I took this blurry picture but hadn't included it before. The 'bulkhead' looks more like an annular bulkhead from the front end. From the back it is a minor inner shoulder.

Yes, there are only 3 holes, so no possibility to insert a rod for leverage.

 

[popgun pete]

Very close in your analysis!! However, there is what I called a 'bulkhead' inside the nose cone. The back end of the head piece/muzzle sits close or against the forward side of the bulkhead, the barrel o-ring seats against the rear side of that bulkhead. In this fashion, all the pressure sealing occurs at the nose cone, and none is needed for the muzzle piece. I took this blurry picture but hadn't included it before. The 'bulkhead' looks more like an annular bulkhead from the front end. From the back it is a minor inner shoulder.

Yes, there are only 3 holes, so no possibility to insert a rod for leverage.

Three muzzle relief ports is less than the usual complement of four muzzle relief ports for Mares pneumatic spearguns. Maybe at one stage of its development Mares were considering using six ports, but were concerned about a potential loss of muzzle integrity over time with a virtually all-plastic muzzle. I have attached a close-up of these muzzles from the "Skin Diver" advert back in 1981 (it appeared over a number of issues at the time). Until you posted your photos I had not seen or heard anything more about them for 30 years! The advert photo does not show much, it is hard to make out any ports even though the main image is deliberately focusing on the muzzle end of the guns. The text of the advert says nothing about the muzzles at all. At the time I wondered if there were inclined rectangular ports hidden in the cylindrical section behind the buttresses which acted as a roof over the top of them, but now it seems the real reason was to lower the production cost of the muzzles. Production plastic injection moulds are costly, so this design would have been an expensive exercise for Mares which did not eventuate in long term savings for the company.

If there is a shock absorber in the nose of the muzzle it can be hooked out with a right angle-ended probe from the rear end or it can pushed out working from the front once you get the tip of the probe into the circumferential gap between the muzzle body and the inner part of the shock absorber. That gap is what allows the shock absorber to move as the surrounding rubber sleeve absorbs the piston impact.

 

[popgun pete]

If there is a shock absorber in the nose of the muzzle it can be hooked out with a right angle-ended probe from the rear end or it can pushed out working from the front once you get the tip of the probe into the circumferential gap between the muzzle body and the inner part of the shock absorber. That gap is what allows the shock absorber to move as the surrounding rubber sleeve absorbs the piston impact.

On muzzles with screw on nose caps the shock absorber inserts from the front end as it trapped between a step constriction that is just in front of the muzzle relief ports and the innermost projection of the nose cap that pushes against the front end of the rubber shock absorber sleeve when the nose cap is tightened up. This system cannot be used on one-piece muzzles, so the shock absorber inserts from the rear end, as would be the case with this moulded nylon muzzle which appears to have a longer than usual and somewhat tapered nose end.

 

[popgun pete]

Interesting that I should search out this thread when looking for bore sizes in the "Mirage" when a gun has become available with the 3 port plastic muzzle also described on this page. The gun is a "Ministen", the shortest of the "Sten" guns that for a period in the early eighties used this buttressed plastic muzzle, although in the USA this same model is called the ".Shortie". Mares did not persist with the new muzzle for very long and probably prayed that no one using one decided to lean on their hand pump and explore high pressure shooting which may have done for the weaker muzzle.

 

[popgun pete]

The "Ministen" arrived yesterday, but I only got around to unpacking it this morning. It is the single power version with the grey colored tank and is a “Ministen” rather than a “Shortie” which is the US market version of the gun.

Interesting to finally see firsthand and hold a gun that I have waited nearly 40 years to see. The plastic muzzle is clearly to lower the cost of production, but despite being fattened up with a buttress section to increase its strength and only having 3 muzzle relief ports instead of the usual 4 the large screw thread that holds the muzzle on the inner barrel nose will be plastic and under high shooting pressure you might blow the muzzle clean off the gun stripping out all the plastic threads.

The gun should be lighter in the nose in the water with this muzzle fitted, but not by much and Mares evidently decided to discontinue it. Significantly Mares never fitted the all plastic muzzle to the longer "Sten" models as maybe the risks of failure there would be much higher, especially as users explored what volume of air that they could shove into their “Sten” gun with the hand pump and still load it. Often gun instruction were left unread and discarded with the gun’s packaging as using the gun seemed completely obvious. Thus cautions and warnings often went unnoticed.