You can get some awesome props (and semi-functional weaponry) for LARPing.
Case in point, this 10-shot repeating crossbow which fires rubber tipped bolts. I picked this one up yesterday, and can fire off a bolt every two seconds (I could probably go faster, but I don't want to break it). It's got decent accuracy up to about 6m (20ft), and could seriously poke an eye out if fired point blank.
Or, you could suitably repaint a nerf gun as an alchemical/black-powder projectile.
Items like this are relatively cheap, if you know where to look, but they could seriously unbalance a game if handled incorrectly. I got this for much the same price as a foam sword just under a metre long. Being able to shoot at a swordsman without them being able to get a swing in is a serious advantage...being able to fire off half a dozen shots in the time it takes them to close in on you, that could be devastating.
A single (skilled) marksman with one of these could take down two or three armed foes before they get the chance to strike, but is that fair? Probably not. Is it realistic? Actually, it probably is based on various sources I've been reading.
Weapons like this don't take a lot of skill to operate, but could easily be incorporated into a game in a more balanced fashion in a few ways...
They could be expensive. Getting a weapon like this to function in a real world sense (rather than a cheap Boffer version) would take intricate skill and a degree of craftsmanship far beyond a mere bow or sword. Making the weapon far more expensive than a sword would certainly limit it's spread, so would making the ammunition expensive (in which case the shots might be used sparingly, testing for bolt damage when the ammunition is plucked back out of victims). If a weapon such as this proved too cheap (or too expensive) for it's relative effectiveness, a freemarket economy might allow fluctuating prices until the weapon settled into a price zone suitable to it's potential.
The weapon could be fragile, with intricate parts it might be prone to breaking down (and thus requiring mechanical repairs at the end of each conflict like armour). Swords may lose their edge over time, but the maintenance on obscure weapons may limit their use to characters who have specialised in them.
Or maybe the weapon is dishonourable to use, characters wishing to ascend the ranks of status and nobility might find such weapons distasteful and might not even want to risk working with people who use such weapons due to the attached stigma.
Possibly a combination of these, or something else entirely.
Just something else to think about.