Here's a PDF on the subject from ArmaLite's Technical Library: (they do this on both their AR-10 and AR-15 bolts.
TECHNICAL NOTE 13: IMPROVED RIFLE BOLT
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
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. U.S. patent # 5911173 issued on June 8,
1999 and U.S. patent # 6044748 issued on April 4, 2000 (both issued to Mark Westrom
and assigned to ArmaLite®, Inc.) protect these significant improvements to AR-15 style
© 1998 ArmaLite, Inc. All rights reserved. Reformatted 110404.