Magazine Checks

Dusting off a few proven techniques for improving accuracy.

by Joe Carlos

When I came on as Armorer for the Army Reserve Marksmanship Program I machine rest tested 135 National Match uppers and only 39 shot MOA. The Reserves didn’t previously have an in-house maintenance program and lacked parts so I substituted sweat (mine!) and eventually cut group sizes in half. When I left the Team a decade later all 135 of those uppers shot sub-minute groups with some type of ammo if tested properly. This article is not so much about shrinking groups as getting all of your groups (rifle or pistol) to hit to the same point of impact (POI.)

Magazines, integral or detachable, work by using a spring to push a fresh round up in front of a bolt or slide. That cartridge is then pushed into the chamber either manually or by a compressed spring. The magazine under the bolt is exerting upward pressure on it. If one magazine exerts a different pressure on the underside of a bolt or slide it can result in POI changes as compared to other magazines. Most magazines have feed lips, followers, and springs which influence feeding and hence the orientation of the cartridge in the chamber and the pressures that the bolt or breech face, extractors, and ejectors exert on the rims of casings. Those differences can also influence down range bullet impacts. High capacity double stack magazines feeding rounds alternately from right to left at an angle, such as the M1A and AR-15 rifles, likely have the potential to exacerbate differences in orientation and pressures on case heads. There have been firearms that fed from the side. Most of these were military guns the ultimately fell out of favor. I have never heard of this design causing POI problems but many of them have been submachine guns used in relatively close quarters.

I said that magazines work to change down range bullet impact in part by varying upward pressures that all of them exert on the bottom of the bolt carrier. Locking a charged magazine in an AR-15 with the bolt forward demonstrates this, as you’ll notice the carrier move upwards just as the magazine locks in place, compressing the top round downward. When the top round is positioned on the left the carrier will also want to rotate clockwise a bit but it’s unlikely you’ll be able to see it. When the top round is on the right the rotation will be opposite. You can also perform this test with pistols and see the back of the barrel or the entire slide react. There will be more movement in rack grade pistols as match pistols usually have most visible slop removed by the builder. That doesn’t mean that upward pressures can’t influence in POI in accurized pistols, however.

Read more in the December 2015 issue.

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