The main purpose of gas vents in a rifle barrel are to drop the pressure behind the bullet just before it exits the muzzle. Any pressure remaining when the bullet exits the muzzle accelerates the propellant gas into a jet that adds its impulse to the recoil of accelerating the bullet. It is called rocket effect, and in high power rifles it is a very significant portion of the total recoil.
Opening vents at the muzzle bleeds off the pressure rocket effect needs to work. This is why a multitude of holes that are uniformly drilled all around a muzzle brake and venting perpendicular to the barrel axis will reduce recoil; it does not depend on having the holes slanted or pushing the muzzle down to counter a recoil moment. For accuracy, an even surround of holes has the jetting gas ports neutralize each other's push, and thus avoids contributing to barrel vibration. It is, however, louder for the shooter than vents that slant forward.
Vents that slant forward still produce some rearward thrust. Slanting forward is a compromise, therefore, and you have to make a personal choice about where the balance lies for you? Other slots are cut only on top to help thrust the muzzle down against recoil. Whether that helps more than it adds to barrel deflection and vibration is another personal call. Certainly, in short, rigid guns, such as handguns, there isn't enough barrel length to vibrate all that much, so it usually works out well in that application.
You could talk to the Magnaport people about it.
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Last edited by Unclenick; November 25, 2008 at 09:03 PM.