The barrel and receiver design determines what can be shot from the Model 870. All barrels are clearly marked with chamber information located about half way down, opposite the ejection port. Receivers are marked with letters at the end of the serial number that determine ejection size.
The following is true for all Model 870 shotguns:
V -- 12ga -- 2 3/4" Only
M -- 12ga -- 2 3/4" or 3" Only
A -- 12ga -- 2 3/4", 3", or 3 1/2"
W -- 16ga -- 2 3/4" Only
K, X -- 20ga -- 2 3/4" Only
U, N -- 20ga -- 2 3/4" or 3" Only
J -- 28ga -- 2 3/4" Only
H -- .410cal -- 2 1/2" or 3"
Regards.