Here's a simple mod I did to a store bought toy gun:
I'm surprised I haven't seen more mods of this particular gun online. It's a lot of fun and pretty easy to do.
Toys R Us sells a Nerf-type dart gun called the Air Zone Punisher. (It's not an actual Nerf gun, but a knockoff).
It's a pretty good gun as-is, but it needs a paint job and extended barrels to look more realistic.
Here are some before pictures:
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And the the finished product:
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I extended the barrels with PVC, and unscrewed one of the retaining discs and moved it forward to hold the barrels together (there are two retaining discs that are made up of two parts screwed together, so I split the second one in half, making three discs total. It's very clear what to do when you have the actual toy in front of you).
The PVC barrels are long and just slide all the way inside the existing barrels until they touch the back. I held them in temporarily with black tape, but I may go back and use something that looks a bit better later.
Still from a foot or two away, the tape isn't that noticeable.
(And since these photos were taken I've gone in and added a bit of silver dry brushing to the bullet belt. It looked a little too black next to the weathered gun.)
Here it is in a photoshoot by my friend Adam Chilson:
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The best part about this mod is that the gun still works like new. It's really fun to pull the trigger and watch the barrels spin into action. The dart-firing action is an optical illusion as they fire from a center barrel, not the spinning ones, but when it's shooting you really can't tell.
There have been some excellent miniguns scratch built by members of this forum in the past, and this one isn't as realistic looking as those, but for $50 or so ($40 plus tax for the gun and a few bucks for materials) it's a pretty good option for people that don't have the time or skill to build one themselves.
One more bonus photo:
Here's how the gun looks mounted on the hood of Adam's Mad Max-style battle truck. (Large file size photo)