[jester]

I would think something along the lines of a Mini 14 or a AR 18 would work better than going for a M16, the Mini 14 is forged with a basic Garand style action which is simple, rialable and durable.

Or the AR-18 which is cool, alot of it is a stamped reciever again simple to make with less resources.

I would use a gas system similiar to a M14 or a FN/FAL to deal with the varied powder.

As for the catapult, just load it with a basked full of fist sized rocks so when it flies the rocks fill the sky and land. On massed troop formations it will do damage and break bones.

As for grenades, fulminate of mercury <like in caps> can be the detonator, make the "grenades" into dart like shapes with a weighted end where your "cap" igniter is thus setting the bomb off.

Another option, regular grenades in a jar or even a can with the top cut off, pull the pins, let the jar or can hold the lever down. When the thing flies it will come loose and well you will have a good number of airburst explosions <more effective than impact for troops> or when they hit the ground the jar breacks, letting the lever fly or is tossed from the can again letting the spoon fly, end result lots of explosions.