Starter Shotgun
-5 bullets per shot
-3 damage when upgraded (total 15)
-4 blasts per clip
-slightly more spread
M3
-4 bullets per shot
-3 damage when upgraded (total 12)
-3 blasts per clip
-slightly less spread
There's a slight advantage with the regular shotgun. It can shoot more bullets than m3 and does slightly more damage. It can also shoot 4 times instead of 3 times. Both have the same rate of fire. The spread isn't a huge difference, but the regular shotgun has slightly more spread than m3.
You may want to balance out these shotguns a bit.