SpudFiles

I am not sure that everyone has really grasped the fact that this plug is after the valve.

The plug only has to hold more pressure than the projectile's surface friction in the barrel. It is not a pressurized part. If he is breach loading the ammo is not going to fit super tight in there.

I am still leary of the rubber stop caps. I agree they would but a good bit of pressure outward on the pipe and fitting. Over time this could cause problems.

I would personally use a cleanout cap. You are not likely to wear out the threads unless you are seriously abusing the thing. As for cleanout caps being DWV, I use DWV for silencers and other parts after the valve and have never had any problems. Do not use dwv on the pressure side of the valve though, ever. At least if you like your cannon to stick around

I agree with William and definitely wouldnt try using these test plugs for anything other than what I intend to use it for which is a simple breach load on a cannon barrel.....If anyone has intentions of trying to use this on a dead headed presurized system like a valve or a chamber I would strongly advise against this......If in fact I do like the performance of the test plug as a breach load , I will keep it as an option for this purpose only.....I would never use anything other than pw-nsf on my cannons though as I rarely have a use for DWV with the 2" diam cannons that I make...My chambers are all fibre wrapped aluminum SCBA tanks.