Prop shotgun?

[lowercase]

a lot of the online airsoft stores will give you a discount if you tell them you're doin a movie and you'll put a special thanks and web address in your credits.
you can also check to see if they have any returns that they are willing to sell. they're usually inoperable but look the part and can be had at huge discounts.
i've gotten guns at 75 percent off that look great but won't fire...which is actually a good thing.

 

[Sniper1154]

Also checkout boneyards from places like Evike.com. I got a LOT of my prop airsoft guns (full metal) from boneyards for a fraction of the price. A lot of selling a shot with a gun is getting it to look like it weighs as much as its real counterpart. Whether you go the spring airsoft route or something a bit higher end, I'd suggest filling it with fishing weights or something so that the actor/actress handles it like a real gun.

 

[arroway]

If you had the cash, these would be the best option: http://www.modelguns-worldwide.com/shotguns.htm