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wayouteast how are you going to handle battery swaps? If you're going to make the bottom of the grip removable, you might want to put the battery snap down at the bottom, this way you can get it on and off easier without tugging on anything. Just a thought!
Balanced is something this thing isn't going to be. No matter what, you've got a lot of weight out in front of the grip, between the weight of the revolver cylinder, the barrel, and the rifle receiver and magazine. It's a bit of a pig in real life, and Harrison Ford preferred using the rubber gun whenever possible (remember his wrist injuries in the plane crash) because of that weight.
Granted, we're not making real ones, but it is pretty bulky - I ended up adding about 4500 steel BBs to mine. It'd be the same material as the real thing, with all the air gaps and such so the density should be close. That came out to 52 ounces of weights...the Taurus Raging Judge Magnum (pictured below) runs right around 60 ounces. It's longer, beefy, but doesn't have a rifle receiver or magazine on it; being eight ounces shy of that seems about right. Just about a kilo and a half, so it's certainly in the ballpark.
http://14544-presscdn-0-64.pagely.netdna-cdn.com/wp-content/uploads/2012/12/taurus_ragingjudgea0304111.jpg