I have a British 410 hammer gun with several generations of proof marks, the latest being 1989 vintage. Oddly, the gun is proofed for 2-3/4" shells. I lengthend the forcing cones and shoot standard 3" shells through it. The gun has an enormous and deep pit on the outside of the barrel under the forend. I am certain the pit pre-dates the 1989 proof. I've probably shot 3-400 3" shells through this gun without problems. The reason I'm bringing this up is that .410's typically have much higher chamber pressures then larger gauge guns.
Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)