So you caught the broadside of your buddy's helmet with your HK UMP GBB SMG during some intense close-quarter battle action and now the bolt doesn't quite cycle with a snap like it used to. Never mind the hurt you just brought to your battle buddy's brain bucket, you've got a gun to fix! Whatever you do, don't recycle it. Rebuild it! Pick up one of our HK UMP GBB Rebuild Kits. It's got all the parts you need to bring back the joy you experience when firing your gas blowback UMP on full auto with reckless abandon! Please shoot responsibly, though. Seriously.