Chamber Length Question

[Durby]

my recently acquired Montgomery Ward Western Field is only marked with those words, a serial number, and "12". It does not appear to have a model name, just Western Field. It is not marked with chamber length. The gun is 80 years old. I was wondering how far in the forcing cone is for 2-3/4 inch shells, and how far for 3 inch. I don't even know when they came out with 3 inch shells, but I want to know for sure that the gun will only shoot 2-3/4 shells. So my thought was that the forcing cone location will tell me chamber length?
Thanks

 

[RNB65]

An 80 year old gun can only be used with 2 3/4" shells. 3" shells weren't introduced until the 1960's I believe.

 

[Dave McCracken]

The 870 3" Maggie Numb came out in 55. Few 3" shotguns were around before WWII.

Just use 2 3/4" shells. The shell length is for the opened case.