Been trying to design a light gas gun. I have many years of experience as a machinist (proper one, CNC programming, manual mill and lathe, etc) and I've been working for three years now as a gunsmith at a smaller outfit that makes rifle actions from scratch among other things. I'm not an engineer so excuse the idiocy I'll obviously suffer from, and as a courtesy I've converted some stuff to metric.
This is mostly for a fun project. At this point, I'm planning to make a simple single shot 50 BMG action that is used as the piston portion. This is not a hard part as we already make actions from billet. The piston would then be compressing hydrogen to pop the bust disc valve and send a 5.6mm bullet (probably something like a 5.8 grain VLD or a Barnes copper solid if a normal lead core copper jacket doesn't survive) down a barrel as fast as it can suffer. Being in the design phase, I'm not sure how to go about estimating the length of the piston bore. I'll modify the case to be straight, so just a cylinder with straight walls unlike the bottleneck shape of the regular 50 BMG. This will give me an effective internal piston diameter of 19.4mm (as case wall thickness at the mouth is 0.5mm and a straight walled 50 would be 19.9mm in external diameter).
Long story short, I want to get the most out of the piston. Pmax is around 4100 bar for a 50 BMG case, and the hydrogen would be at 200 bar. To get the most out of it, I want to be able to estimate how long the piston has to be if it's to compress the hydrogen to the point where compressing it any further would stop the piston (equilibrium), taking into account the piston's inertia. For now I'm trying to get ballpark estimation as I know it'll require a lot of tuning for handload (powder charge, weight of the piston) and burst pressure for the disc. I can definitely deal with interesting powder load stuff, but the math to get there escapes me a little.
This is mostly for a fun project. At this point, I'm planning to make a simple single shot 50 BMG action that is used as the piston portion. This is not a hard part as we already make actions from billet. The piston would then be compressing hydrogen to pop the bust disc valve and send a 5.6mm bullet (probably something like a 5.8 grain VLD or a Barnes copper solid if a normal lead core copper jacket doesn't survive) down a barrel as fast as it can suffer. Being in the design phase, I'm not sure how to go about estimating the length of the piston bore. I'll modify the case to be straight, so just a cylinder with straight walls unlike the bottleneck shape of the regular 50 BMG. This will give me an effective internal piston diameter of 19.4mm (as case wall thickness at the mouth is 0.5mm and a straight walled 50 would be 19.9mm in external diameter).
Long story short, I want to get the most out of the piston. Pmax is around 4100 bar for a 50 BMG case, and the hydrogen would be at 200 bar. To get the most out of it, I want to be able to estimate how long the piston has to be if it's to compress the hydrogen to the point where compressing it any further would stop the piston (equilibrium), taking into account the piston's inertia. For now I'm trying to get ballpark estimation as I know it'll require a lot of tuning for handload (powder charge, weight of the piston) and burst pressure for the disc. I can definitely deal with interesting powder load stuff, but the math to get there escapes me a little.
