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Tech Note

May 7, 1996


SUMMARY: The ArmaLite AR-10B rifle bolt bears an unusual improvement which may be mistaken for a defect: the locking lug opposite the extractor is deliberately relieved at the rear so as not to contact the locking lug of the barrel extension. This feature strengthens the bolt.


1. ArmaLite bolts bear 7 locking lugs, each 22.5 degrees apart from the next. At this angle there should be 8 lugs in a circle, but one lug has been removed to provide space for the extractor.

2. The missing lug creates an asymmetry, or imbalance, in the bolt. When the rifle is fired the pressure of the cartridge presses the bolt rearward, and the lugs transfer this energy to the matching locking lugs of the barrel extension. The bolt flexes at the rear inside corner of the lug during this process. Because one lug is missing, the deformation is greatest in the area of the missing bolt. A series of engineering analyses have disclosed that the lugs on either side of the extractor each receive 40% of the recoil load.

3. Relieving the rear of the lug opposite the extractor "balances" the loading of the bolt by restoring symmetry. The load placed on the bolt is spread more evenly over the six remaining lugs. Peak lug loading is reduced.

4. Relieving the lug opposite the extractor reduces peak loading on the lugs neighboring the extractor from 40% to 24% each. It is not reduced to 16 1/3% (1/6 of the recoil load) because a small asymmetry remains: the lugs neighboring the extractor are slightly undercut for the extractor, which results in continued relative weakness at those lugs. Nonetheless, the load on the weakest lugs is reduced 40%, compared to the standard M-16 design, by spreading a portion of that load to the other 4 lugs.

5. In addition to the relieved lug, the AR-10B bolt bears tapered lugs with wide roots. This puts added strength exactly where stresses are at their greatest. The AR-10 bolt is significantly stronger than the similar M-16 bolt. A patent covering both bolt and barrel extension relief is pending.

Copyright 1998 ArmaLite, Inc.

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