If your bbl is stamped "1912" it's definitely 2 9/16 assuming 16ga. According to Madis, a 2 3/4 conversion does not affect value as many factory conversions were conducted.I’m wondering if that may be the tie-in as the Model 12 was originally known as the Model 1912. I know when I found out about 2 !/2” chambers, I pulled the barrel off mine ran the caliber down the chamber until I hit the throat and sure enough, short chambered. I never did have problems with ejection but then the big diet I fed her was cheap blasting birdshot for blasting blackbirds in the sunflower fields.
It just occurred to me, my cracked case issue may be due to not fully seating the rd before firing. It happened once while hunting and twice shooting clays. IDK. She kicks like Nancy Wilson in the '70's and is 103yoa.