The following is a quote from the answer I got from an expert in the U.S. when I asked him the same question:
16ga & 20ga. the early model 12's all came with short chambers!!!! 2 1/2 for the 20 and 2 9/16 for the 16 ga. no chamber length is stamped on the barrel or receiver (near the serial number) A lot of them were opened up to 2 3/4 some by the factory and some by gunsmiths, the factory would use the roll die and restamp the barrel where a gunsmith would steel stamp 2 3/4 in most cases. Only the guns opened up by the factory are worth the money. (the above is for the field grade with a plain barrel). 1927 was the year Winchester started with 2 3/4 in chambers in the 16 & 20ga. There is a small window here 1927-1930. The guns still came with a Nickel Steel barrel, and a factory 2 3/4 in chamber.
Barrels for 20 ga
20ga 30 in barrel is the rarest
28in with a Mod choke is the most common, followed by Full choke
26in after 1925 is not too common
25in is what was first offered on early guns dicontinued in the mid 1920s
Any 16 or 20 ga gun with a Solid rib or factory Vent rib is rare.
As for convincing you that you should want one, all I can say is model 12's (especially in 20 guage.) are nothing but POS. If you know where there's one in good shape, you should tell me where so I can get rid of it for you!
