Go to any custom game with a shotgun. You can have unlimited ammo if you want.

First get the shotgun and fire it until you have one shell left.

Use the combination to lower you weapon, which is the following: LB, RB, L thumb stick, down on the d-pad, "A" button and hold that.

That is the combination to lower your weapon.

Since you have used most of your shotgun ammo, the lowering weapon combination will start to reload your weapon because it includes the reload button "RB".

When you are reloading, the lowering weapon combo will keep the shotgun in reloading position until you throw a grenade or shoot.