Detachable magazine accuracy issues were first noticed in the mid 1960's by master class service rifle competitors using well rebuilt M14NM's in competition. When the US Army and Marine Corps shops finally worked out all the bugs with that rifle and using the best lots of M118 match ammo, commercial .308 Win. match ammo or handloads, they noticed a zero shift of 1/2 to 3/4 MOA at 200 and 300 yards depending on the magazine used. Any magazine that had the slightest slop in its fit would be given to the combat troops; only magazines that fit solidly were used in competition.
Clamped in an accuracy cradle used to test rifles and ammo for accuracy, tight-fitting magazines produced better accuracy than loose one. Not all that hard to figure out why when one realizes that all the parts of a semiauto rifle have to go back to exactly the same position with the same force for best accuracy to happen. With two tight fitting magazines, one would shoot just as accurate but the group center would be 1/2 MOA or more away from where another tight-fitting one would shoot. The rifles recoil was sufficient to dislodge a loose fitting magazine enough that one could see the shot string on target move around in impact. Magazines that fit really tight would stay in place from shot to shot. Sometimes a magazine would be "spread" out at its top end to make the sides bear harder against the receiver to keep it in place. Top level shots could tell the difference shooting the rifles in normal positions shoulder fired, too. All the parts in a rifle have to go back to exactly the same place after reloading the next round if best accuracy is the objective.
For slow fire matches where rounds were loaded one at a time into the attached magazine then the bolt tripped to chamber it, an extra tight fitting one would be used. As the M14 magazine was also used as a palm rest for standing position, it had to be a really tight fit with some extra force required to lock it in place. That also prevented it from being displaced in theprone position by accidental body and/or sling contact. The slow fire magazine was usually marked to indicate it was for slow fire with an "S" or "SF" for identification.
For rapid fire matches where 2 rounds in one magazine and 8 rounds in another would be used for 10-shot rapid fire matches, two magazines had to be used which both enabled the rifle to shoot to the same place. These magazines typically fit a tiny bit less tight so they could easily and quickly be removed and replaced in the reciver but still held immovable in place. One was marked "R1" and the other "R2" (or just "1" and "2") indicating one was for rapid fire and the first one holding 2 rounds, the other was the second one holding 8 rounds.
In a conversation with GySgt Frank Kruk, USMC, in the late '60's (1964 NRA Nat'l Service Rifle Champ) he told a bunch of us Swabbies that the Leathernecks on the USMC Rifle Team would almost fight over who got what magazine when a crate of new ones was opened for replacing their old ones. We jokingly suggested to him that the USMC team go back to M1 Garands and get them from the USN shop that built them as they had no issues with all sorts of 8-round clips and we didn't care how they fit. Garands had no accuracy issue with those clips. GySgt Kruk said that would be fine with him 'cause he knew the Navy Garands shot 7.62 NATO rounds just as accurate as their M14NM's and were much easier to maintain.
Regarding commercially available detachable mag's fit for standard factory bolt guns, I don't think they're any different. Put one in and if it has any noticeable slop in fit by moving it around, set it aside and try another one. Once you've got a couple of really tight fitting ones, test your rifle for accuracy shooting it as best you can. . .without a magazine. After putting 5 to 10 shots (whatever a magazine holds) in a downrange group, put in a magazine and shoot the same number of shots. See if both groups have the same center; last one's well centered on the first one. Note also if the rifle shoots to a different point of impact with different magazines. If you can shoot that rifle no worse than 1/3 MOA at 100 yards without a magazine in it, you should be able to see the differences. The less accurate the shooting system is, the harder it will be to see how magazines impact zeros and accuracy.
Those for high end tube guns are much better as well as more expensive. And bolt action match rifles with a 5-round clip guide in the receiver bridge never had any accuracy problems with them charging internal box magazines.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master
Last edited by Bart B.; May 15, 2013 at 07:53 AM.