Have an Uzi of any kind and another weapon. Get a motorcycle and stand next to it. Select the Uzi. If you have more weapons, select one next to the one you want to hold on the bike. Then, press Y to get on the motorcycle. Just before the radio station starts, quickly press White or Black to switch weapons. If done correctly, you should have the weapon equipped and be holding on the bike. You cannot fire the weapon while on the bike. This may require a few attempts. The best weapons to use are the AK47, M4, and the shotgun.