FWIW I "tested" the recoil attenuation as best I could by shooting my 6920 one day, installing the FOSSA that night, and then shooting the next day.
The recoil reduction was significant enough that I noticed it immediately. I can't say I paid any attention to how it affected the muzzle rise specifically, just that I had an instantaneous reaction of "oh wow it does actually work some".
I have since bought a couple more and "one of these days" intend to have the gunfighter 1 cut off one of my 14.5" BCMs and install the FOSSA so I can do a side-by-side comparison.
I also bought and installed one on a 16" .308 build that I have also run a KAC MAMS on. Recoil reduction was noticeably better on the MAMS, but (and this goes for the 5.56 version too) the FOSSA doesn't generate much if any pressure blast to the sides/rear. On the ground between the concrete walls/legs of the rather narrowly spaced shooting stations the MAMS blast muzzle blast was slapping me so hard I started to feel like I was getting a concussion whereas I never noticed the FOSSA.
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