I don't split them in my inventory.
I start with the one stack of 100 ammo in my hand, then:
(All clicks are Control+Right Click, unless stated otherwise)
Click 1: Turret 1, give it 50, I have 50.
Click 2: Turret 2, give it 25, I have 25.
Click 3: Turret 3, give it 12, I have 13*.
Click 4: Turret 4, Ctrl+Left Click, give it 13, I have 0.
Click 5: Turret 1, take 25, it has 25.
Click 6: Turret 5, give it 12, I have 13*.
Click 7: Turret 6, Ctrl+Left Click, give it 13, I have 0.
Click 8: Turret 1, take 12, it has 13*.
Click 9: Turret 7, Ctrl+Left Click, give it 12.
Click 10: Turret 2, take 12, it has 13*.
Click 11: Turret 8, Ctrl+Left Click, give it 12, I have 0.
End result*:
T1: 13
T2: 13
T3: 12
T4: 13
T5: 12
T6: 13
T7: 12
T8: 12
Player: 0
As close as you can get to evenly splitting, without ever splitting a stack in your inventory, and without ever opening a gui.
*not sure about rounding, could be other way around
