Sherman Bell published this study of GaugeMate adapters back in 2006. It would make sense that 16g to 12g bore would not diminish, and possibly increase, fps
28g Gold chamber adapter in a 16 gauge gun:
(Both Remington and AA factory loads were nominally 1200 fps)
A Remington 28-gauge STS factory load of 3/4 ounces of #8 lead shot, gave a velocity (at 4 feet) of 1215 ft/second and developed a pressure of 5100 psi at the front of the forcing cone.
A Winchester 28-gauge AA factory load, also with 3/4 ounces of #8 shot, gave a velocity of 1252 ft/second and developed a pressure of 5800 psi.
