I was just wondering if this was a S&W Factory port job or were they offering to have the gun MAGNA-PORTED.
NO. The 686P just has a slot in the top of the barrel and the front sight is backed up behind it.
It really is a redirection of some of the exhaust gas that drives the bullet out of the top of the barrel. It acts like a jet engine, pushing down the barrel against the rise caused by the gun recoiling back toward the shooter.
The problem is that the upward directed gas provides a microscopic effect in neutralizing muzzle rise which actually is the result of rearward directed recoil force causing a rotation at the wrist.
Compensators actually reduce muzzle rise (and felt recoil) because the compensator has an area where the gas going forward expands and slams into a "wall" creating a forward force which offsets part of the rearward recoil force. The gas from the xpansion chamber does exit vertically but that very small force does not provide significant reduction or change in muzzle flip.
I believe this topic was covered by Michael Plaxco in his book Shooting From Within, and he did some experiments modifying compensators showing this effect. Good book BTW.
IMHO, getting a hole cut in the top of your barrel is a waste of money..... and I do own several ported barrels, so it's not like I didn't try to be a believer. It just doesn't work, and Plaxco's experiment showed why.
From the book, pg 150:
"A common misconception is that the reason a compensator works is due to the jet propulsion of gasses escaping upward out of the port to drive the muzzle down."
He goes on to describe how to prove it by modifying a compensator: opening up (increasing the area) for the upward gas ports does not show any significant change in muzzle flip, but increasing the front hole increases muzzle flip because it reduces the effectiveness of the gas going forward.
