As far as I've seen the Zombie Raid gun handles are original and have never been used for any other game. The shotguns have a pretty heavy duty metal casing and the handles are plastic so one of the 2 are gonna give out and with the mounted guns being slammed around in the wild the handles are going to take all the abuse. When I bought my Zombie Raid one of the original handles was missing and it bugged the sh!t out of me. I did the research looking for replacements and you are basically left with only being lucky enuff to find a leftover gun. From what it looks like, many other ZR owners are looking for replacement handles also.
Since I had no luck finding a replacement I decided to just make my own set. The game takes place in 1918 and I always thought the more modern looking assault handles looked out of place so I hand carved a set of matching wooden handles in a more antique style and stained them. They're smoother than the originals and they match pretty well with the cab art/theme. Plus they're basically just a bolt-on replacement.
I thought about making sets of these to sell but they can be pretty time consuming with all the shaping and sanding. The handle half is easy enuff it's the angles of the inner half that can be tricky. I might eventually get a few sets made up if the interest is there.