You Sir, are correct. On my 3" Mag M12 "Heavy Duck" I did a through "mike" job, and then pattern tested it with both AA Trap loads and some Hevi=-Shot for later on Mallards (also a Doctorate on Airborne barn pigeons with the AA's) The chamber ring is about 5/16" on the 3" mag-and the receiver is about 3/64" thicker in wall thickness (at the ejection port side-than my three other std. 2 &3/4" chambered 12 ga. Model 12's- The ejector spring is about 0.0035" thicker- and the firing pin is also NOT interchangeable with the std. 12- but the trigger housing group and action slide release are- The wall thickness is of course, heavier at both receiver and at the muzzle-with appropriate taper on the even 30" from the rear of the threaded barrel-and the face step- to the crowned (very slightly) muzzle- Nom. Bore dia. is still .730" "on the dime" and the choke indictates .030 Constriction- patterns like a champ-even and full, makes you wonder how a guy could miss- the lead plug was removed from the rear buttstock group cavity hole (parallel to the stock bolt through or access hole-I like the slight "muzzle forward balance of the M12" so with the heavier barrel wt. over the std. field 12's- I'll gradually add some more wt. (lead shot encapsulated in Acra-Glas in a tube I'd guess- and see how she goes. I've seen folks "measure" the barrel on an assembled M12 and come up with a odd or fractional reading- most accurate way is to dis-assemble the gun into the two basic groups, and hook your Stanley tape on the muzzle and read it right at the end of the receiver end threads at the "step" Dead Nutz every time!!
