Re: new at reloading
The BOSS system is a muzzel mounted tuning device invented by Browning that is used to nulify barrel harmonics, or at least adjust the harmonic nodes to match a particular ammo/load to increase accuracy. It is a break or cylinder on the muzzel that screws toward the end or toward the receiver, with visual graduations so the shooter can make finite adjustments.
The concept is to tune the barrel to the ammo, which is the inverse of what we do when we develop a hand load for accuracy. In the latter case, we adjust bullet selection, seating, powder, and primer all to find the "sweet spot" in barrel hamonics so that primary and secondary harmonics are neutralized to some extent, and to get the barrel to always be in the same vibrational mode as the bullet leaves the muzzel. Hence a repeatable impact point, and what we call "accuracy".
The supposed advantage of having the BOSS is that you can tune the gun to shoot (consistent well made) factory loads like well developed hand loads.