Hi
I have not really built the kit, so I can't speak intelligently about it, but I can give you some ideas
It may take some patience but you may have to carefully chop the hand and carve out a small portion to "seat" the handle of the machine gun. then you will have to add the part of the hand you chopped and make up for the gap with some sculpting to make it truly look like the hands are gripping. Either that or you can drill a tiny hole in the middle of the gripped hand, cut the handle of the machine gun and make it fit with the hand. then glue/sculpt any remaining part that does not look complete.
It may be scary to chop up important parts of the model
but if you take your time it should turn out better than before. So this is just a couple ways I might do it, other more experienced modelers may have another idea