The word "gun" itself seems to work. Package this as a game and you've got a pretty fun game on your hands :)

Gun related stuff works: bullet, holster, barrel

Other stuff that works: key, door, lock, smooth

Some words that result in "flintlock": violence, anger, swing, hit, impact

Well that's easy, subtract "gun" :P

hammer - keyboard = hammerhead

Makes no sense, admittedly!

- dulcimer and - zither are both in firmly in .*gun.* territory it seems..

Bullet

hammer - red = lock

hammer + man = adult male body (75%)

if I'm allowed only 1 something, I can't find anything either, if I'm allowed a few somethings, "hammer - wine - beer - red - child" will get you there. Guessing given that a gun has a hammer and is also a tool, it's too heavily linked in the small dataset.