My primary hunting rifle for the past 6 years has been a MKII BAR in .270win w/ BOSS. Let me first say this Browning is very accurate, up there with bolt guns. 5/8"-7/8" groups at 100 yards consistently year after year.
As others have indicated, yes the ported BOSS does add muzzle blast however the advantage in recoil reduction and faster followup shots outweighs that (at least for me). I have tried a few setting on the Boss system. The first of which was the factory recommended "sweet spot" for the brand and bullet weight of my chosen cartridge. Groups were mediocre with this setting. Interestingly enough the best groups I get, with my chosen cartridge, is setting the BOSS to "0" and the micro-adjustment to "2".
Anyways, that's my personal experience with the Boss. I love the reduced recoil and if I'm honest, I think it looks pretty cool too
