Not strictly rocket science, but my little brother made himself a .177 cannon as a metalwork project at school (it was supposed to be a mantelpiece ornament, but what the heck).

We tried setting it off with the powder from a penny banger. The results were disappointing. With a hair-thin touch-hole, the thing refused to fire; when the touch-hole was bored out to about half the calibre of the gun, there would be a "poof" which left us blinking dazzled for several seconds. Meanwhile, the .177 airgun pellet which we used as ammunition remained firmly in the barrel. After we grew bored with extracting the pellets from the barrel, we concluded that there was some critical minimum aperture width, based on grain size or whatever. We thought that boring out the cannon to .22 calibre might have some effect, but the parental and educational authorities had by now twigged the significance of the drill bit sizes my little brother was using.

Friends of ours discovered that secretly removing an inch or three from the stick of a large firework rocket could sometimes enliven firework displays; as could surreptitiously leaving unopened tins of baked beans among material being collected for large local bonfires.