11mm vs. 13mm? (Dry Muzzle, Same Outer Barrel Size)

[Diving Gecko]

Hi guys,
This might be in the vast treasures of DB somewhere, but just to "cut it out in cardboard" - as the Danes say, I'd like to ask some specific questions.
I have discussed this a bit with a fellow Dark Sider from here and I would just like to swing it by you.
I remember reading that 13mm inner barrels have the upper hand on dry muzzled guns but now, I am a bit puzzled.

Say, we take two Stens of the same length (both with a 40mm outer tank), one with an 11mm inner barrel and the other with 13mm, add dry muzzles to both of them - then how would they compare to each other in terms of getting the most power out of the guns?

I know a 13mm will have a more powerful shot if the two were at the same pressure, but the 11mm would also be easier to load.

To get equal power out of an 11mm we would have to raise the pressure, which would also make it more difficult to load.
I guess my question is whether, at the end of the day, if we want the same power of shot from each of the two, will the loading effort will be the same and the only difference is the pressure of the guns?
Let's say the loading effort of a 13mm gun is X and the spear leaves the gun with Y energy - would those two values be the same in an 11mm gun?
(Remember, I am not an engineer, so this might be completely flawed)

If that is so, then I guess there are a few pros and cons to take into consideration:
- An 11mm gun will constantly operate under higher pressure. That might not be an issue with a Mares, but as far as I remember Seacs are rated to 20bar whereas a Mares' goes to 30bar.
The trigger pin of a 11mm gun would be slightly harder to press.

- The piston will be lighter in the 11mm. My feeling is that it is a negligble pro as the spear weighs relatively much. But maybe it has a say in how the piston slams into the bumper?

- It has been pointed out that a 13mm inner barrel has the advantage that one could use both 7mm and 8mm spears with an 11mm inner barrel can't accommodate the 8mm spear.

 

[tromic]

...
Let's say the loading effort of a 13mm gun is X and the spear leaves the gun with Y energy - would those two values be the same in an 11mm gun?
...

I would say you were right. For vacuum barrel these two values (X, Y) would be (almost) same for 11 mm piston. Maybe neglectable lower value for Y for 11 mm barrel. Difference might be because of difference of weight for two pistons per force acting on their cross section.
This ratio is better for 13 mm piston, if both pistons are same design (same steel part, only plastic part OD different).

 

[Jegwan]

The maximum loading effort for one person using the same spear could be considered a constant.
The maximum loading effort is an expression og the maximum stored energy in the gun after loading. So the maximum stored energy will in principle be the same.

If the maximum loading effort not is restricted by the persons technique or power but due to spear flexing, I would believe that you can obtain a higher maximum loading effort with a thicker spear.

The area of the 13mm piston is in theory approx. 40% bigger than the area of the 11mm piston. Does this mean that the pressure in a 11mm gun need to be 40% higher - or am I missing something?

If so; will a 13mm gun @ 18bar equal a 11mm gun @ 25bar.


Jégwan

 

[tromic]

...
The area of the 13mm piston is in theory approx. 40% bigger than the area of the 11mm piston. Does this mean that the pressure in a 11mm gun need to be 40% higher - or am I missing something?

If so; will a 13mm gun @ 18bar equal a 11mm gun @ 25bar?


Jégwan

Yes, that is right.

 

[Jegwan]

So; calculating backwards from the maximum aloud pressure in a Sten...
You will (using the same spear) theoretically benefit from using a 13mm barrel version of the Sten if you can load it when it's pressurized to 22bar or more... ;-)

On top of that comes the considerations regarding the trigger pull, pressure and weight of the piston/gun.

Jégwan

 

[popgun pete]

It all comes down to the cross-sectional area of the piston. With a larger cross-sectional area there is more force for a given chamber pressure and hence more energy for the shot, but a bigger muzzle loading effort to go with it. A smaller diameter piston has lower friction due to the smaller circumference of the seals running on the matching inner barrel wall. Studies have shown that a 10 mm inner barrel ID is the most efficient.

An 8 mm diameter spear of the same length has 30% more mass than a 7 mm diameter spear, so to throw it from the gun with the same level of acceleration you need 30% more force. A 13 mm diameter inner barrel has 40% more cross-sectional area than an 11 mm diameter inner barrel, so it has the capability to accelerate a heavier shaft. So it is not just that a shaft diameter will fit inside the inner barrel, but that the gun has the energy to propel it at sufficient velocity. The chamber pressure could be increased, but it is more convenient to stay within a certain range of pressures that will not require a stronger standard of construction at a higher manufacturing cost. Plus lighter guns, as modern pneumatic spearguns are, have the advantage of floating after the shot.

 

[Diving Gecko]

Just a thought - if one uses freeshafts with no slider on the tail end, I guess one could still use an 8mm spear in an 11mm gun, right?

 

[tromic]

Just a thought - if one uses freeshafts with no slider on the tail end, I guess one could still use an 8mm spear in an 11mm gun, right?

Right! But it is not convenient because the pressure should be over 30 bar. I had tested TombaF800 on Cyrano 1100.

 

[Diving Gecko]

Right! But it is not convenient because the pressure should be over 30 bar. I tried TombaF800 on Cyrano 1100.

True, for a second - after grasping it all - I completely forgot about the higher pressure needed in an 11mm, duh;-)

So, for say a 7mm spear at the end of the day, it actually does not make much difference at all whether you go with an 11 or 13mm gun as long as you stay within the max pressure limits.

You mention that a 13mm piston is more efficient than an 11mm at the same stored energy level (13mm piston = (11mm + 40%)) because of its weight? I guess the bigger mass stores more energy, right? But I guess it also takes more energy to get it moving? And Pete points out that a 13mm piston has more friction than an 11mm one, so again, perhaps it evens out between the two?

Also, I think I remember you working on reducing the weight of a piston once, right? Was that primarily to stop it easier with less noise and less risk of damage when it hit the shock absorber?

Furthermore, while the mass of the piston and friction of it might be lesser issues (especially the latter), I really think I would favor a 13mm gun because with equal power to an 11mm as the 13mm would have a smoother/lighter trigger (40% lighter also?).
After shooting my Seac 13mm gun at 20 bar, I can honestly say that even a 2mm trigger pin is a bit too hard for my liking. My gun mounted video clips confirm that I do deflect the aim a bit when pulling the trigger.
(I have taken the trigger bushing/pin apart and it's tricky, but I really hope Marko can make a 1.2-1.5mm pin mod for me. Stay tuned)

But perhaps we should make a note to potential modders looking for the most power that perhaps it would be smart to not choose Seacs if you are looking for an 11mm gun to be modified with a dry muzzle. That is if I am correct in that Seac "only" has a 20 bar limit.

 

[tromic]

True, for a second - after grasping it all - I completely forgot about the higher pressure needed in an 11mm, duh;-)

So, for say a 7mm spear at the end of the day, it actually does not make much difference at all whether you go with an 11 or 13mm gun as long as you stay within the max pressure limits.

You mention that a 13mm piston is more efficient than an 11mm at the same stored energy level (13mm piston = (11mm + 40%)) because of its weight? I guess the bigger mass stores more energy, right? But I guess it also takes more energy to get it moving? And Pete points out that a 13mm piston has more friction than an 11mm one, so again, perhaps it evens out between the two?

Also, I think I remember you working on reducing the weight of a piston once, right? Was that primarily to stop it easier with less noise and less risk of damage when it hit the shock absorber?

Furthermore, while the mass of the piston and friction of it might be lesser issues (especially the latter), I really think I would favor a 13mm gun because with equal power to an 11mm as the 13mm would have a smoother/lighter trigger (40% lighter also?).
After shooting my Seac 13mm gun at 20 bar, I can honestly say that even a 2mm trigger pin is a bit too hard for my liking. My gun mounted video clips confirm that I do deflect the aim a bit when pulling the trigger.
(I have taken the trigger bushing/pin apart and it's tricky, but I really hope Marko can make a 1.2-1.5mm pin mod for me. Stay tuned)

But perhaps we should make a note to potential modders looking for the most power that perhaps it would be smart to not choose Seacs if you are looking for an 11mm gun to be modified with a dry muzzle. That is if I am correct in that Seac "only" has a 20 bar limit.

Piston should be as light as possible. Piston without skirt, with O-ring only has lower friction. Friction on shot is much lower than while piston is moving with low speed (during loading).

With 1.6 mm trigger pin instead of 2 mm, on 20 bar, force on pin would be 0.35 kgf instead of 0.64 kgf. Not a big difference! To this force due to pressure you should add a force of friction(s) (O-ring, sear lever tooth to piston mushroom) plus force of spring against sear lever, together more than 1 kgf.

 

[Diving Gecko]

Piston should be as light as possible. Piston without skirt, with O-ring only has lower friction. Friction on shot is much lower than while piston is moving with low speed (during loading).

With 1.6 mm trigger pin instead of 2 mm, on 20 bar, force on pin would be 0.35 kgf instead of 0.64 kgf. Not a big difference! To this force due to pressure you should add a force of friction(s) (O-ring, sear lever tooth to piston mushroom) plus force of spring against sear lever, together more than 1 kgf.

Ah, I could not re-do your calculations before the edit but I think it was because you added in the spring force and a guesstimate for o-ring friction as well, right?
Basically, I just want the lightest possible trigger but I also want to make sure there is enough spring tension to enable a secure hold of the sear onto the piston.

 

[Jegwan]

You mention that a 13mm piston is more efficient than an 11mm at the same stored energy level (13mm piston = (11mm + 40%)) because of its weight?

I understood tromic's statement as the 13mm piston has the highest "area to weight ratio" - and that is true... but a relatively lighter piston (11 or 13) is both benificial for the speed and the noise - like you and tromic already have indicated.
I believe that this is the reason why the designer of the Taimen decided to make the piston rod in titanium.

Regarding the conclusion about 7mm spear and 11mm and 13mm barrels:
I would like to repeat that your Seac with 13mm barrel at 20 bar will equal a same size Sten 11 at 28 bar.
I personally find it difficult to load my Sten 11's above 28 bar with a 7mm spear due to the spear flexing... So I would have a hard time loading a Sten 13 above 20 bar with the same spear... Maybe I haven't trained it enough..

The trigger pin in the new Stens is 1,5mm and the sensitivity can be adjusted (It's recommended not to do so in the manual - So be carefull and think safety if anyone plan to do so).

Jégwan

 

[Diving Gecko]

I understood tromic's statement as the 13mm piston has the highest "area to weight ratio" - and that is true... but a relatively lighter piston (11 or 13) is both benificial for the speed and the noise - like you and tromic already have indicated.
I believe that this is the reason why the designer of the Taimen decided to make the piston rod in titanium.

Regarding the conclusion about 7mm spear and 11mm and 13mm barrels:
I would like to repeat that your Seac with 13mm barrel at 20 bar will equal a same size Sten 11 at 28 bar.
I personally find it difficult to load my Sten 11's above 28 bar with a 7mm spear due to the spear flexing... So I would have a hard time loading a Sten 13 above 20 bar with the same spear... Maybe I haven't trained it enough..

The trigger pin in the new Stens is 1,5mm and the sensitivity can be adjusted (It's recommended not to do so in the manual - So be carefull and think safety if anyone plan to do so).

Jégwan

Ah, "area to weight ratio" makes perfect sense.
Speaking of shock absorbers. Why do almost every manufacturer insist on the crazy design of a nylon piston hitting a nylon/Delrin bushing at high speed and then the bushing pushing a rubber piece? Evo Air actually seems to have improved on that with turning the parts around. In their design, the piston hits the rubber which is guided by the Delrin bushing. Seems quite a bit more smart.

I, too, have trouble loading my Seac 90 (6.75mm spear) at 20bar. With more experience and perhaps a 7mm spear, maybe I could get to 21-22 bar.

The trigger pin on the Seac can be adjusted too, but it is not the sensitivity but the stroke - how far it travels before release. I'll try to figure out what it really is on the Sten.
Also, the force exerted on the trigger pin by the internal pressure of an 11mm gun is 40% higher than on a 13mm gun - but as Tromic points out, the lion's share of the total trigger force needed comes from the sear lever spring and o-ring friction.
Still, in my quest for the lightest trigger this should steer me towards 13mm guns.

I do think my next gun could be a Sten 13mm (with 1.5mm trigger). Or perhaps a second hand Airbalete. Or something homemade... or nothing;-)

 

[popgun pete]

True, for a second - after grasping it all - I completely forgot about the higher pressure needed in an 11mm, duh;-)

So, for say a 7mm spear at the end of the day, it actually does not make much difference at all whether you go with an 11 or 13mm gun as long as you stay within the max pressure limits.

It makes a difference of 40% more energy for the shot and about the same increase in effort to load the gun, so the spear will be accelerated to a higher velocity to exit the gun from a 13 mm inner barrel compared to the 11 mm ID version of the same gun.

 

[Diving Gecko]

It makes a difference of 40% more energy for the shot and about the same increase in effort to load the gun, so the spear will be accelerated to a higher velocity to exit the gun from a 13 mm inner barrel compared to the 11 mm ID version of the same gun.

Yes, I am with you on the 40% potential power advantage. But that advantage only comes out if both guns have the same pressure in them, right?
What we were trying to figure out is precisely what practical differences there are between the guns if an 11mm has 40% more pressure in it.
Basically, the two guns should then shoot the same size spear at very close to the same speed, right?
The only difference would be the weight and friction of the piston.

Then comes secondary factors such as an 11mm having to withstand higher pressure and having a higher trigger pull.

But what about the compression ratio (not sure it is the right term but the ratio between the tank diameter and piston diameter), how does that manifest itself as it is different between the two?

 

[Jegwan]

The devil is in the detail

The 11mm gun will in principle perform the best if the maximum loading effort when the piston it locked at the trigger and the spear is the same on the guns. This because of the less decreasing pressure ramp (see Peters graph) and due to lower weight and friction of the piston.

But I don't know how much these factors have to say in practise - they might be negligible compared to the force released.

Jégwan

 

[tromic]

Yes, I am with you on the 40% potential power advantage. But that advantage only comes out if both guns have the same pressure in them, right?
What we were trying to figure out is precisely what practical differences there are between the guns if an 11mm has 40% more pressure in it.
Basically, the two guns should then shoot the same size spear at very close to the same speed, right?
The only difference would be the weight and friction of the piston.

Then comes secondary factors such as an 11mm having to withstand higher pressure and having a higher trigger pull.

But what about the compression ratio (not sure it is the right term but the ratio between the tank diameter and piston diameter), how does that manifest itself as it is different between the two?

13 mm barrel gun would shot more powerful because of higher compression ratio. To shot same, 11 mm barrel gun should have a bit more than 40 % higher pressure (42.8 %).

To be more clear:



If loading effort were same when started loading, at and of loading 13 mm piston would require about 4 % more force than 11 mm piston. There is only about 2% more stored energy with 13 mm piston than with 11 mm. This is due to different compression ratio (1.13 / 1.09)

 

[mikeles33]

Hello i am close to buy a mares sten 11 and i saw this conversation..
What we are saying is that 13mm guns are more powerfull than 11mm and in order to be the same a 11mm must be pressed with 40% more air?
The 13mm uses 8mm spera and the 11mm a 7mm so dont you think that the diference is covered by the less mazz of the spear?
So isnt possible the same guns under the same pressure but with the stock sets of spears for each would shoot the same?
Surely using 7mm spear the 13mm will have advantage but we must know that the 11mm will need less effort to load as they say..

Definitely i dont know which gun to choose...!!!!

 

[Jegwan]

What length and what and where do you hunt? Any spear diameter preference?

 

[mikeles33]

What length and what and where do you hunt? Any spear diameter preference?

Mostly white fishes 0.5-3 kilos at 1-3 meters and sometimes until 6 meters.Usually i will wait fish to aproach and i would prefer a gun of 70-84cm pneumatic of course.