M16/M4 reliability problem, increased bolt friction causing jams





The interactive transcript could not be loaded.



Rating is available when the video has been rented.
This feature is not available right now. Please try again later.
Published on Aug 18, 2008

DESCRIPTION / updated May 2010:
AR15 modification chart:

Regarding the functional reliability of the AR15/M16 system there is one problem I have never seen mentioned anywhere - increased bolt friction.

On its forward move while feeding the next cartridge the bolt has to overcome the friction between the specific mag lip and the cartridge below (depending on how much the mag spring is compressed).
The bolt carrier is pushed forward by the bolt spring and the bolt head hits the next cartridge in the magazine. The bolt head is now trying to move rearwards (resistance/drag of the cartridge in the mag) and to make its turning move but it can't.
The head of the cam pin is pressing against the left inside of the upper receiver (until cartridge is released by the magazine).
Thus pressing the bolt carrier in the opposite direction causing the bolt carrier to twist clockwise creating additional pressure on all bearing surfaces. With just a small amount of dust/dirt there it slows down the bolt which may not have enough speed and power anymore to close completely. Because at the last few millimeters the bolt has to overcome the power of both the extractor and ejector spring.

And it seems as if the designers in the mid-sixties were aware of this problem.
Therefore the forward assist is located at the right side. So the forward assist not only generates forward pressure but also a counter-clockwise strike at the bolt carrier to rattle it free.

Actually most of the other contemporary designs using AR18-style bolts (G36/XM8, FN SCAR ...) show the same built-in flaw. But don't suffer so much from this early turn matter due to the different shape of the bolt carrier, different location of the contact area cam pin/receiver (9 vs. 12 o'clock) and clockwise locking turn.


Why proved the HK416 to be more reliable?

Larry Vickers:
"HK 416s utilize a 20-percent stronger buffer spring, which produces more bolt velocity."
The stronger buffer spring (more momentum on the forward move) is needed to compensate the higher impulse generated by the gas piston system.
Additionally the HK416 uses a "spring-powered" bolt head.
Means the bolt head is pushed forward in the bolt carrier.
This works against the drag force of the magazine and forces the bolt head out of the "jammed" position.
The AR15/M16 bolt head stays in the slightly rearward "jammed" position (on the forward move of the bolt) due to the friction caused by the gas rings.
This spring serves as a rebound spring for the firing pin as well to interact with an internal firing pin safety (apparently this became neccessary due to the higher closing energy of the bolt carrier).
But this is not the only reason for it's implementation otherwise that spring would rest against the bolt carrier and not against the bolt head.

The AK bolt head (like some other designs) is not forced to rotate earlier than needed to lock.
The bolt head is pushed forward by the "stop edge" (yellow) of the bolt carrier.
The bolt head is forced to rotate if it's reaching the little ramp (encircled yellow). This rotates the bolt head enough that it's guide cam will pass the stop edge and will be now forced by the bolt carrier to continue it's rotation.
If an earlier try to rotate (like at the AR15/M16) is not an issue - why did they design it that way?

Try it (no risk to pinch your finger):
The upper won't move or sag a bit and stop then.

April 2009:
There are recently a few discussions going on about „The Groove"—Theory or "the cam pin groove thing". It's about wear marks in the upper receiver caused by the cam pin.
And there you can find the following answer:

Sent: Friday, March 27, 2009 9:21AM

The groove you are seeing is completely normal. And is common among even DI rifles. It is a result of the cam pin acting on that surface during the feeding operation. It is not dangerous and it will stop eventually.
Joe Devens
LWRC International

And here are some very nice pics by LWRC that show the interaction cam pin / guide channel in the upper receiver:

May 2009:
POF-USA Roller Cam

January 2010:
LWRC Advanced Cam Pin

March 2010:
The Big M4 Myth
Check out: Here are the findings of my testing:


When autoplay is enabled, a suggested video will automatically play next.

Up next

to add this to Watch Later

Add to

Loading playlists...