This should not consume ammo for the second shot. The rate of fire dosent really make it good enough to be burning 2 ammo with a 25% loss on damage. It also causes weird issues like -1 ammo. Which causes the reload to have 1 less than it should.
Just make it fire a second "bonus" bullet. You could even take the % reduction a little higher and it'd still be good due to the overall ammo you would have.