Added version 1.7- You can now purchase upgrades for the minigun, flamethrower, GL G20, and shock hammer! I'd have added more, but I've reached the entry limit in weapon_upgrades, it seems. Here's what the upgrades do (as of 1.8):
Shock hammer:
Lvl2: Explosions electrocute npcs, slightly larger radius.
GL G20:
Lvl2, 3 & 4: Increased magazine capacity and range.
Flamethrower:
Lv2 & 3: Increased range and damage.
Minigun:
Lv2 & 3: Slightly increased damage, decreased bullet spread, increased magazine capacity.
Sonic Boom:
Lv2: Increased range, decreased charge time and re-fire time.