Skeeter is correct kind of. Winchester originally chambered the model 12's in 20 gauge only and in 2 1/2" chambering only. The time frame is off a bit. The short chambers changeover happened early. Then in 1914 both 16 gauge and 12 gauge chamberings were offered. The 12 gauge was offered in 2 3/4" chambering only. The 16 gauges were chambered in 2 9/16" length only. Then in 1927 Winchester changed the chamber length on all gauges to 2 3/4". I have not heard that 16's were harder to convert to the longer shells than the 20's. To me it would seem the opposite since the 20 had shorter shells, but I have never heard anything about either conversion, I just know it was a common practice.

POORDEVIL mentioned that his early 16 shoots 2 3/4" shells well and ejects well. He does not believe it was altered. It is easy to check. Remove the barrel and use a straight edge stainless ruler. Simply push the ruler down till it touches the forward edge of the chamber, then check the measurement.

Many people do not realise that shotgun shell are measured when the hull has been fired. A 2 3/4" loaded shell actually only measures @ 2 1/2" loaded. When the crimp expands on firing the hull had a place to "get out of the way". Twelve gauge guns with 2 3/4" chamberings can quite often chamber a loaded 3" shell. When shooting a 3" in a shorter chamber will not have a place for the lengthening case to get out of the way. The shot column must squeeze past the hull.