In one of Sherman Bells "Finding Out For Myself" articles published in Double Gun Journal he found that shooting shells longer than the chamber increased pressure by about 300 PSI. That is a negligible amount. I shoot shells that are 2 3/4" in 130+ year old Parkers with Twist Steel barrels without incident, however I shoot modest velocities (1,150 fps) and light loads i.e. 10 gauge 1 1/8 oz of shot, 12- 1 oz, 16- 7/8, 20- 3/4 oz. I do not hesitate to go up an 1/8 oz occasionally, but don't do it when I shoot 100+ rounds a day.
Harry
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