if your just shooting skeet use #7's or 8's .....hunting birds I use #6's.
Finally, is there any time of ammo I can't use in this gun? I understand i need to keep it between 2 3/4 & 3"
Might only take 2 3/4 , I have an older model 12 winchester and it's only chambered for 2 3/4. And yes the M on the barrel is for modified. If its an older gun , it wont take "screw" in choke tubes. Modified is fine for clays & hunting.