The BOSS is a muzzle brake that can be click adjusted to tune barrel vibrations. All barrels vibrate upon firing. Wherever the muzzle is as the bullet exits is where the bullet goes. So controlling the vibration so the muzzle is at the same place in the vibration sequence every time for a particular load will improve accuracy.

By adjusting or tuning the BOSS, you can dial in the sweet spot for a particular load. I have a Winchester M70 with a BOSS. Winchester put the BOSS on some models for a few years back in the 90's and I got one in 30.06. I use it mostly for whitetail but it is my backup elk rifle. So I have a setting on the BOSS for my 150 gr deer loads and one for my 180 gr elk loads. This setting is repeatable so all I have to do is adjust the scope.

Browning offers the BOSS on their bolt and BAR models. They also offer a BOSS without the ports so it isn't a muzzle brake but still offers the barrel tuning. The ports do soften recoil but add a lot of noise and muzzle blast. Also, if you are shooting prone or on a bipod, they can kick up a lot of dust and debris.

RE: All those concerned about the shorter action and lighter weight of the 300 WSM vs the 300 WM. I think if an inch or so of rifle length and maybe 6 ounces of weight makes a big difference in how well you can carry or swing an 8 lb rifle, maybe you should work out or consider an easier sport . Smart a$$ mode off.