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AR-15/M-16 Armorers Update

The following information is for approved and certified department AR-15/M-16 system armorers. If you don’t have the skill and training to correctly disassemble and reassemble any duty weapons, do not do so.

We began a recent patrol class by inspecting all of the rifles, which were of the AR-15/M-16 type. We discovered that one of the rifles’ internal parts had been taken out and reassembled incorrectly. On the street, this would obviously have disasterous consequencs. For a basic user of the AR-15/M-16 type rifle or carbine, there’s no need to remove the parts from the lower receiver. However, for an armorer responsible for complete periodic cleaning and assessment of the weapon, there’s the needs to fully understand how to do so correctly.

Over the years of instructing Patrol Rifle Instructor, Basic Skills, and Armorers classes with some of the most knowledgeable people in the business, we’ve found AR-15/M-16 type duty weapons disassembled for cleaning and maintenance are often reassembled incorrectly.

My co-author is one of these top gunsmith/armorers, Ned Christiansen of Michiguns Ltd. He recently produced a cutaway AR-15 lower receiver to illustrate the very real and dangerous problem of improper hammer-spring assembly. Improper assembly may create a situation in which the patrol rifle/carbine will either not fire, or fire several times and then fail. This has vital consequences on the street when life is at risk.

The lower receiver attaches to the stock and pistol grip, and it contains the magazine well and the fire-control parts. We have officers practice the disassembly of these parts in our classes, provide numerous warnings on this issue and demonstrate through photos and hands-on practice to prevent them from incorrectly assembling the spring and placing it in the lower receiver. The following photos and descriptions illustrate this problem.

Department armorers for this weapon system must know this issue in detail and learn the proper assembly/disassembly procedures. Use a checklist each and every time you do any work of this type to prove you’ve followed the correct sequence.

Photos 1 & 2
In Photo 1, the AR-15/M-16 hammer spring is installed backwards. The first problem with this is that when the hammer is cocking, the spring unwinds instead of winding up, greatly reducing hammer-spring tension and leading to a high probability of misfires.

The second problem is that the trigger pin will “walk” or move out sideways. The rearward-extending legs of the hammer spring act as a detent for the trigger pin by snapping into a groove on the pin. In this condition (shown in Photo 1), the hammer-spring legs are held from contacting the trigger pin by the trigger body. The spring legs look like they’re lying on top of the trigger pin, but they’re just missing it. When (not if) the trigger pin moves sideways, the trigger falls off sideways on an angle, which often produces several rounds of automatic fire and then a shut-down. You’ll see the trigger pin protruding from the side of the receiver as it is in Photo 1.

Same thing from the right side. See the white paint line on the side of the hammer? That’s to indicate where the top portion of the spring should be on the hammer. (This line is not cheating, it’s making darn sure yourgun works!)

In these two photos, we can clearly see the hammer spring is assembled incorrectly. Note the trigger pin walking out to the left side of the lower receiver.

Photo 3
In this photo, the hammer spring is installed correctly on the hammer, but its spring legs are not resting on top of the trigger pin, preventing it from coming out. Instead, the long legs of the spring are resting under the trigger pin on the bottom of the receiver. The result: The trigger pin will move sideways as in the above scenario, resulting in unwanted automatic fire and shutdown.

Hammer-spring legs should be clearly visible from the top and seen resting cleanly on top of the trigger pin, spread all the way out against the receiver walls.

Incorrect assembly happens when the assembly sequence is done out of order. The trigger should go in first. That makes it almost impossible to get the hammer-spring legs under the trigger pin.

Photo 4
This photo shows the hammer cocked without the hammer pin installed. This isn’t really a problem; we’re just showing a technique for re-installing the hammer.

You can actually cock it by hand prior to putting the pin in. This takes the spring pressure off and makes it easier to eyeball the alignment for getting the pin in.

A J-shaped length of spring wire permanently staked into the hammer acts as the hammer-pin holder or detent. If you look into the hammer’s pin hole, you can see this J spring partially blocking the pin hole (not visible in the photo). When you push the pin into the hammer-pin hole, the spring flexes a bit, and when the pin is all the way home, the spring snaps into the hammer pin’s middle groove, holding it from moving or walking out. Note the hammer pin pictured by the trigger, with the groove in the middle and one on the end; the hammer’s J spring snaps into the middle groove to hold the hammer pin in place.

Hammer and trigger pins are identical, interchangeable and can be installed from either side. The outer groove is for the hammer-spring leg to lock into to hold the trigger pin in place.

Also, in this photo it’s clear the hammer spring is on the hammer right because the top portion lines up with the white line.

Photo 5
This is pretty common. The hammer spring is installed correctly on the hammer, but one of its legs is not lying cleanly out against the receiver wall and on top of the trigger pin. Instead, it’s inboard and lying in the trigger-spring’s coils. Because the trigger pin is grooved on only one end and not both, this gun now has a significant chance of going full auto in the next and then shutting down. It would’ve been better had these pins had been designed with a groove on both ends that would give us a 50-percent greater chance of beating Murphy’s Law in this area.

The left-side leg (at the bottom of the photo) is right where it should be. Both legs must be in contact with the trigger pin to ensure proper function. It’s easy enough to do check your street guns now to make sure.

This article first appeared in the Illinois Tactical Officers Association’s newsletter, ITOA News. Thanks to Sabre Defence for contributing a lower for the cutaway.

Ned Christiansen is a gunsmith and the owner/operator of Michiguns Ltd. Visit his company’s Web site at

About The Author


Jeff Chudwin is the Law Officer Tactical Ops columnist. He’s also the 2009 Law Officer Trainer of the Year. He retired as chief of police after 38 years of service for the Village of Olympia Fields, Ill. A founding member and current president of the Illinois Tactical Officers Association, Chudwin is a former assistant state’s attorney and has been a firearms, use-of-force and emergency response trainer for more than 25 years.

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