Rem Express Magnum
Maybe I'm missing something here but I just bought a Rem Express 870 and on the barrell is stamped 20 Ga 2-3/4" or 3" . Even older shotguns (1960's and beyond) had a data stamp saying 2-3/4" before 3" shells came out. You could measure the chamber length however if yours is missing this stamp altogether I'd take it to a gunsmith. It may be old and not meant for modern shells or it some after market barrel made in China.
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