My guess, as I own and shoot 7 Model 12's- ranging from a skeet grade M12 28 gauge to several 3" Mag 12's-- the plastic hulls didn't really get rolling until the late 1950's, from 1912 to about 1957- paper hulls were the norm- and the softer opened up crimp folds most likely cleared the front radius of the receiver ejection port when the gun was cycled. The oldest M12 I own and shoot is a 1921 mfg. field grade 12 gauge 28" full plain barrel nickel steel (same analysis basically as the later Winchester Proof Steel-that was just another marketing ploy by WRA--) and it has 2&3/4" chambers-std. for the 12 gauges since 1916--
A hunting pal has a 16 gauge field grade made in the same time frame as my 12 nickel steel M12- no chamber length marked, he shoots Federal 16 plastic hulls in it all day long, no problemo.