I dont know about the GPMG but I've had a 30 cal, 50 cal and bazooka from Crossfire and they have been good quality as far as replica's go. I went to collect the guns and they are a proper engineering company. I know they also supply some of the current army regiments for recruiting purposes. Also spotted on the credits on guy martins ww1 tank that they supplied the replica Lewis guns. I guess they can't be that bad!
I've heard worrying things about MM but by and large good things about Crossfire. A few of my mates have bought from them and they seemed happy with what they had. But they are replicas so don't expect like for like quality against a deact.
Just my thoughts.