I have a 2 9/16" singleshot Iver Johnson that replicates the chambers in two damascus sxs guns. I also have a Pressure Trace piezio transducer pressure measurment instrument and a chronograph. The Iver is instrumented and serves as my test gun for pressure testing.

After reading Sherman Bell's article where a true 2 1/2", short forcing cone, chamber was used to test 2 3/4" shells, I went forward with the knowledge that he got a maximum of about 15% increase in pressure after testing a variety of loads, and some had no increased pressure. The average increase in pressure seemed to be about 10%.

I shoot full 2 3/4" paper Fed shells loaded to generate about 6500 psi in the short chambers. The loading manual I have suggests this load should generate around 7000 psi. That loading manual refers to a 2 3/4" chamber pressure gun.

I've shot hundreds of these loads thru my damascus guns. When in doubt, get real data. Everything else is heresay. If someone has direct knowledge that this is an unsafe practice to shoot 2 3/4" shells in a 2 1/2 -2 9/16" chamber, please share.