Modular guns-recipe concept

The idea is that you can re-configure the gun by swapping out parts, but
that swap is time-consuming and requires both skill and the proper
equipment to get right.  The guns are pretty good but not perfect, and
can't be tacticool'd out.

Numbers here are not set in stone and frankly I've not cross-checked
them because I'm thinking they're likely to be changed as firearm fans
check in.  So let me know what you think of the details, please.