I havn't found a way to drop any guns. You have a holster to your left and right that can store a weapon. This allows you to carry 4 guns at any one time. If you want to get rid of it you can just holster it. If you find a new weapon, you can pick up this weapon with the hand that you are holding the weapon you want to drop and the weapons will swap over. This is the only way you can remove a weapon from your inventory in Arizona Sunshine.