Tweak some recipes and stats of homemade guns, add m2 rifle modification

Should be ready for commit, but if anyone sees anything that needs further tweaking, let me know.

The blunderbuss in particular has received a large buff in the form of actually having the potential to hit a target. This should make it somewhat more useful, although it was an excellent noisemaker previously.
Lenses shouldn't be required to making a homemade .308 rifle, even if it says "a basic scope". Maybe add a "makeshift scope"? I would prioritize making a gun over making a scope or sights for a gun in a survival situation.