Try a dab of clear cement on the gun grips? I know my two MG gundams (Freedom and Destiny) both have slight ridges on the palms to hold their weapons, and I could see how that would make it difficult for them to hold something meant to lie flat against the palm. Otherwise, the only Gundam I have "outsourcing" its weapons right now is my girlfriend's 1/100 Heavy Arms, which is carrying some sort of BFG from the Eva Unit 01 Experimental kit, but that's a different problem...