Karl,
The tests done by Sherman Bell and published in the Double Gun Journal were with a 12 gauge. There was up to 300 psi increase with the 2 3/4" shell in a 2 1/2" chamber. The 2 3/4" shells from RST are 200 psi less than the 2 1/2" shells. If you shoot the 2 3/4" shells you need to see if the ends are fraying. This will be a sign they are too long for the chamber area. Then you probably want to use 2 1/2" cartridges.
Hopefully, Drew will chime in.
Ken
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