SpudFiles

I was thinking of building a spring gun for shooting bbs. Would the design be better off with a longer spring to have more room to compress or a shorter or stiffer spring with less room?

a long "soft" spring would probably last longer, but not shoot very hard. a long stiff springs would work great, but you would need a strong housing. my gamo pellet gun had a very ridged spring that is probably 8-10 inches long. the housing is reasonably thick steel and the spring is only compressed a few inches, if that.

I was also thinking of having a 3/4 end cap with o rings to fit in a 1.25 or 1.5 inch pipe. Would it be better to have a charging handle or a piece of thin wall pvc as a pump?

When it comes tp springer power, it's all about swept volume - so if you're using a short and stiff spring, make sure you chamber and piston are large in diameter.