So you finally made it to a carbine course. You and your buddies have spent hard-earned cash to attend an urban rifle course taught by professionals. The first couple hundred rounds go pretty well, but halfway through the course your pet M4 starts choking. Occasional short stroking on a training range is an annoyance. On the street or on the battlefield, it could mean you're dead. Let's look more closely at this problem and a solution.
The AR-15 is a gas operated rifle, and leakage at any point in the gas system may cause short-stroking. One possible source of leakage is the bolt carrier key. The key is attached to the carrier with two 10-32 socket head cap screws. If these screws begin backing out, the key will become loose and gas will bleed out before it enters the carrier. This scenario is easily preventable.
Check the carrier key screws for tightness. If they are loose or can be easily removed, then go ahead and remove them completely and degrease the screws. I keep a small jar of lacquer thinner on my bench for this purpose.
Apply red Loc-Tite to the clean, dry screws and reinstall. Military manuals specify 35-40 inch pounds of torque to tighten. Some gunsmiths recommend a higher torque. Just make sure they're nice and tight.
To properly stake the screws, you will need a MOACKS from Ned Christiansen at Michiguns, Ltd. MOACKS stands for "Mother Of All Carrier Key Stakers." They're not kidding. This multi-purpose tool should be in every armorer's kit. Slide the carrier into the MOACKS and center the carrier key screws between the pointed MOACKS screws.
Tighten the four screws until they bottom. Remove the MOACKS and inspect your staking job. Steel displaced by the MOACKS against the screw heads make the chances of the screws coming loose about as likely as Sarah Brady entering an IPSC match.
File off any burrs on the sides of the key caused by the staking operation. Most major manufacturers are doing much better staking keys than a decade ago, but if the factory stakes look weak a trip through the MOACKS is insurance against failure.
The MOACKS tool from Michiguns in position for staking. It uses hardened screws to displace metal against the key screws, preventing them turning out of the bolt carrier.
The MOACKS tool allows much more accurate staking than can be done with a hammer and punch. This makes the carrier look better and also better secures the key screws.
Gus Norcross, originally trained on National Match rifles and pistols by the National Guard Marksmanship Training Unit, specializes in Garands, M14s and 1911s at his small shop on the coast of Maine. He will be offering gunsmithing tips and tricks on Fridays. His website is AngusArms.com.