To start with lets rember that the BOSS system was used in the 70's and 80's. Expeant accuracy then and now are two different things. Then if you had a sporter rifle that would reliably shoot an 1" group, you had quite a good rifle! Now we expect that.
The Boss system worked by screwing in in or out , like a nut on a bolt.
Browning held a nation wide contest to find the most accrate Boss equiped rifle. Handloading and the Boss the only things that you were allowed to change. Rifles had to be stock Brownings. BBL's were from carbine length to 26" magnums. The boss worked on all of them.
Lots of rifles shot under an inch, one under 3/4 regularly. I don't rember what the winner really was, after all, that was the most part of 40 years ago.LOL!
Would they work, today? with all the advantages we now have? Heck if I know.
Respectfully
Terry