What are the pitfalls to shooting 3 inch magnums in a 2 and three quarter inch chambered JC Higgins Model 20 12 gauge?

Answer 1

The JC Higgins Mod 20 is a High Standard Mod 200, an aluminum frame shotgun, so I see only a couple of problems. The High Standard company went out of business before 3" magnum shells were released to the market, so their shotguns were never designed for the higher pressures of magnum shells. In addition, the longer shell would protrude into the forcing cone at the breech end of the barrel and when the shell was fired, the folded end of the shell would be impeded from unfolding by the forcing cone, resulting in higher pressures. These two factors could well combine to shatter the receiver and blast aluminum shrapnel into your face.

Other than that, no problem.