There's a difference between the amount of ammo you hold and clip size.
You can hold 264 (I think) ammo at most for rifles but the enranged rifle only has a 18 clip size (it can shoot 18 times before it needs to reload) so basically #1 increases from 264 to whatever and #3 increases the clip size of 18 to like (for example) 24.

The only problem i see with #1 is the gun itself dosen't have a specific ammo its just rifle ammo.