TDP castle nut has three staking notches, equally spaced, of 0.084 width +0.02/-0.00 tolerance, at 45 degree angle. There's no debate as to what these notches
are for, castle nuts should be properly torqued (38-42 ft/lb), and
staked in two places. Despite experience and lessons learned from LE
armorers and users, some still do not believe it's necessary to stake
the castle nut.
Military carbines are
delivered with their castle nut staked. It is insufficient to rely on
friction alone to ensure the castle nut's immobility, lest it should
rotate, and in turn, allow the receiver extension to rotate and spill
the buffer retention pin and its spring into the receiver.
So
far as we can tell, the "stakable" locking ring was introduced by Colt
in the mid 1980s with M16A2. The old CAR style locking ring didn't have
staking notches. Considering the AR's buffer and buffer spring are
contained within the receiver extension, it's of understandable
importance that the receiver extension doesn't unduly rotate and loosen,
which would lead to the buffer retaining pin and buffer retaining pin
spring being released into the lower receiver, this invariably results
in the trigger or hammer being jammed.
Staking
the castle nut is the operation where material is moved from the end
plate into the castle nut's staking notches with a mallet and punch,
rendering the castle nut immobile. Staking the castle nut in two places
is specifically referenced in the technical manual (pg 250). A staked
castle nut is not impossible to undo, it is exceedingly difficult to
loosen without an armorer's wrench, or a castle nut wrench, as it should
be.
Staking
can be performed using a center punch, or an auto center punch. The
punch recommended by the AR guru William Larson is the Mayhew 5/32
center punch. It is free insurance against loosened castle nut and
receiver extension.
While we're at a loss as to
the reluctance of users to do it, more puzzling and unsettling is the
trend for manufacturers to market castle nuts and end plates that cannot
be staked, instead, these products either use a set screw or friction
alone, it is our opinion neither should be considered for hard use.
We're not sure of the wisdom of deliberately going against the written
instructions in the TM, and best practice long established by Colt and
FN. The notable exception is Knight's Armament's end plate sling
adapter. According to KAC (we're paraphrasing here), the mating
surface of its end plate and the indentation on the rear of the KAC
receiver is sufficiently precise to have little to no movement, which
doesn't allow the receiver extension to rotate to loosen, there is no
need to stake KAC's end plate/sling adapter.
Our
opinion is if your AR is for sporting and recreational purpose, you may
have more tolerance for improper or total absence of staking, a
loosened castle nut and RE may ruin your day, it won't cost you more
than minor inconvenience. If you have a defense or duty AR, be sure
your castle nut is in spec, properly torqued and staked in two places,
it may well mean the difference of life and death, not something one
should take lightly.