Catastrophic barrel failure possibility? A reason to inspect your guns?

[spoolin01]

After lightly pressurizing a ScubaPro Magnum 105 that I bought but just let sit in my garage for years, I noticed the slightest of apparent leaks from the muzzle. It was so slight I though I'd just finish pressurizing the gun and let it be, but the tinkerer took over and I took the gun apart. Here's what the barrel looked like, just behind the rear o-ring groove.

In some respects, it really looks like a crack, in others not so much. I tried flexing the tube a bit, but couldn't see that the crack opened up. I took some various grits of sandpaper to the tube, and followed up with a polishing wheel. While the surface is mirror-like now and I don't obviously see the crack, there are enough annular scratches left from imperfect polishing that I call it inconclusive so far. I'm going to go back a couple of grits to improve the surface finish, and maybe I'll chuck it in a vise and use some leverage to try to flex the tube, but I don't know if that is likely to show anything if the crack isn't bad enough. I should mention after cleaning the inside of the tube, I don't see any crack peering down the end, but it's 6 inches or more down and the viewing angle isn't ideal. I also dragged a fine dental pick around that area from the inside some, without feeling anything.

Has anyone seen or heard of something like this? Any ideas how to diagnose or best test this? Is there anything equivalent to magnafluxing that works with aluminum? I could just re-assemble the gun, cover it with a heavy tarp, and pressurize it to 30 or 35 bar from a scuba tank, but I'm wondering if there's a way to seal just the barrel tube for testing. That would reduce the explosive force, save the risk of damage to the rest of the gun, and allow me to watch the suspect area while bringing the pressure up. I had thought maybe a heavy rubber or plastic sleeve or plug inside to block the trigger cuts - finding a cap for the muzzle end shouldn't be too hard I hope.

I'm a bit at a loss for what to do next, besides junking the gun. Any ideas?

 

[popgun pete]

Leaks are usually due to rubber seal defects, I have never seen a cracked barrel tube. I cannot say I recognize the parts in the photo, but it has been some time since I repaired my "Magnum 95" version of that gun. Attacking the metal parts is a real "no-no", surface finish is everything in a pneumatic, either where the seals sit or what sections they have to slide over when being positioned on the gun. A cracked plastic nose cone or nose cone seat can leak, plus the nose cone has a different type of seal on a "Magnum" with a tapered face seal rather than an "O" ring. Better look for a schematic diagram for the gun, I remember seeing one some time ago on the Web. You may have damaged the gun beyond repair, but they come up on eBay every now and then, so might be better to buy another one. I bought mine there for under $50 and it only needed a new "O" ring. Being out of production buying one is a bit of a lottery depending on the way that the previous owner treated the gun. The inner barrel is different from that used in a "Sten", Scubapro made a number of changes to the design, so the replacement barrel if you need one has to come from a "Magnum".

 

[spoolin01]

That section of the barrel is just structural, nothing else goes on there, except inside. I've removed the anodizing but if anything it will be even smoother than stock. I don't think I've appreciably changed the o-ring groove dimensions, which shouldn't be that critical, and if I understand correctly, that o-ring is just part of the power chamber isolation, not a seal between inside and the outer world.

I'm more worried about determining if the barrel is cracked. It looked so clearly so, but I'm a bit surprised that the slight sanding and polishing appears to have removed those indications. A tube of those dimensions has got to have a working pressure safety margin of 10X or more from anything the gun should have seen, and a burst pressure well beyond that. Were it defective, though, I suppose it's possible it could nevertheless have cracked.

It's good to hear this may be a fairly unknown risk. I still am pondering ways to convince myself I can trust it.

 

[popgun pete]

I had a closer look at the larger image and I think that these marks are surface flaws in the tubing, not some failure. They may have been there in the tubing before it was processed into the barrel. As the gun gave no problems here, your leak was at the muzzle end, I think that you have nothing to worry about unless you caused some problems with your subsequent removal of material. As it is inside the partitioned reservoir section and the "O" ring will not be nicked when you reinstall it, assuming that you have removed it and smoothed any sharp marks away, then it should be OK. Tubes split lengthwise when they fail and these marks are in a full thickness section of tube, so I don't see that as the pattern of damage which would be caused by the stress of air pressure in the gun.

 

[popgun pete]

Something else occurred to me that you could try. If you load the gun with some pressure inside it (but not too much) and the piston front seal is behind any inner barrel crack then air will flow into the inner barrel and escape from the gun as it will percolate out in front of the piston. I remember on my old Sten that I could feel the piston seal passing through the spot where the partitioning bulkhead was locked in place (two brass half circles instead of a plastic or metal circlip used in later guns) as the gun was being loaded, so the piston seals pass behind that location when the gun is cocked. With the gun not loaded the pressure is the same everywhere inside the gun, so any leak through a crack will not show up.

 

[spoolin01]

That's a great thought about checking for leaks when loaded, that area is definitely forward of the piston then. Once I've finished careful inspection, I'll reassemble and load it after charging it a bit.