My friend just recently acquired a brand new KWA/Umarex H&K licensed USP-CT. However, the gun has been problematic when we tried to shoot it. 1.When we got it to work, gun only weakly blew back once. 2.Barrel jammed w/ three brands of BB'S. He needs help with the gun, and since I have no experience with KWA GBB's, I need help.