Is this the smartgun you guys are talking about?
I looked up the Iron Grenadier figure and I remembered his gun looking an awful lot like this one a guy posted on NECA's facebook
If it the same one the size doesn't look too bad, but I think the length of my custom one is a little bit more to scale. That being said if I knew where to get these iron grenadier figures and if they could be bought for cheap, I'd probably try to hook my smart gun up with better pieces.
The custom I think using the GI Joe smart gun
My custom using some plastic bits and a small scale MG42