Given that it's raining special ammo in the Crucible, every shotgun might as well have infinite ammo there. I think this idea the OP has really won't change much in PvP.

What about if it has 2 shots. Not 2 shots on spawn, not in mag, 2 shots total. 2 shots that fit in the mag, and no other shots. If you screw up any kills, which even really good players mess up sometimes, you will be out of ammo. Scavenger mods won't help much, you need to get those kills. Plus the fact that if you use two shots for one kill, now you only have one shot with no margin of error until you get pickup. I think it won't necessarily be broken.

not exactly, shotties don't always one tap peeps, coming from a somewhat competent shotty user. plus, someone rocking double primaries is a rare sight, meaning you usually get ammo on kill anyway. i think it'll really depend on its stats and perks if it'll be op