Does black powder (gunpowder) explode under pressure?

Very doubtful. There may be a very small increase in temperature due to rise in pressure, but it shouldn't be nearly enough to cause the powder to ignite. Now if you slammed the end cap on very quickly then the chances increase. I don't think paper would prevent anything. You may want to check out "fire pistons" here: a href="http://www.firepiston.com/" rel="nofollow"http://www.firepiston.com//a Disclaimer - I don't recommend anyone try this. I may be completely wrong. ;-)