"We have commissioned independent pressure/velocity testing with Winchester AA and Remington STS ammunition in 12 to 20, 12 to 28, and 16 to 28-gauge adapters.
Pressure testing was done at the forcing cone area. Because different test guns were used, the 12g gun was measured at 3" and the 16g at 3 5/16" from the breech face. There are no industry standards for pressure at that point.
Here is a sample of the data:
16 to 28 Winchester AA -- 5900 psi and 1252 fps
16 to 28-g Remington STS -- 5100 psi and 1215 fps
Nominal velocity for both brands is 1200 fps.
SAAMI pressure standard for 16g at one inch from the breech face is 11,500 psi."
You might contact the GaugeMate folks for the other infro, and please let us know the results.
http://www.gauge-mate.com/index.htm
