Originally Posted by Drew Hause
Sherman Bell published this in 2006, and I'm not aware of any other pressure data. Certainly not unexpected that pressures would be lower, with both a larger bore and some degree of gas escape if the wad does not adequately seal the ejecta.

In the 12g gun with 20g adapters:
A Winchester 20-gauge A-A Sporting Clays factory load of 7/8 ounces of #7 1/2 lead shot, fired in a Silver adapter, gave a velocity of 1319 ft/second and developed a pressure of 7700 psi.
The same 20-gauge load fired in a Go1d adapter gave a velocity of 1324 ft/second and developed a pressure of 7800 psi.
With all 0-rings removed from the Gold adapter, per your request, the velocity was 1302 ft/second and the pressure was 7900 psi.

Recent velocity report
https://www.shotgunworld.com/threads/gaugemate-insert-performance.558220/#post-4854142

Fitting the standard reducers designed for 2 3/4" chambers to shorter chambers does require some grinding

BD: please check your friend's empties for 'feathering' also

Thank you, Reverend.

All the best

Skeeter BD