You need two turrets and C4.

With one turret out and one in your inventory, take out the C4 and stand behind the turret thats out.

Hold X like you're trying to pick up the turret and press right on the D-pad at the same time.

You will pick up the turret and take out the other one at the same time.

Press the right trigger to set one and then press Y to put the other turret back in your inventory and you should have nothing in your hands if done correctly.

Go buy a gun and it will be a third gun instead of replacing one you have.

This can be repeated and is easier on single player but can be done on two player.
Do this close to the gun crate and be careful not to pick up any ammo or you will have to start over.