Blr243 wrote:For yonks looking at adi reloading data and when choosing which powder to use I notice the highest chamber pressure don’t necessarily give the highest muzzle velocity .....I would have thought higher pressure gives higher FPS It’s about time I knew why is it so , or rather why it is not so ?
I don't know the science, but different powders burn at different rates. If they burn very quickly they run out of steam sooner, if they burn more slowly the pressure lasts longer. If you have a particularly short barrel you'll want a fast powder, for a really long barrel a slower powder so it's still making pressure right up to the bullet exiting the muzzle. .22LR uses very fast powder as it has to work in pistols as well as rifles. By the time it's poking along eighteen inches of barrel all the powder is gone and pressure is dropping, so velocity is being taken by bore friction. Even after all the powder is burned, the bullet still needs about 250fps to actually overcome the bore friction and fall out the muzzle. I guess the actual number can be calculated, but the lowest I've measured a bullet that actually exited is 275fps, anything lower stopped in the muzzle. That was in a .243.