Found this on youtube ...Could this help overcome some of our Modelgun problems 
DESCRIPTION / UPDATED May 2009 (appendix subj. HK416): Regarding the functional reliability of the AR15/M16 system there is one problem I have never seen mentioned anywhere - the self-stopping fea...
DESCRIPTION / UPDATED May 2009 (appendix subj. HK416):
Regarding the functional reliability of the AR15/M16 system there is one problem I have never seen mentioned anywhere - the self-stopping feature of the bolt.
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).
What actually happens is the following.
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 with considerable force against the left inside of the upper receiver (until the 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 jams the bolt on its forward move.
And even if the bolt doesn't jam -- it slows down and 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. Perfect within reach for the thumb of a lefty! Like me.
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.
UPDATED - HK416
Why proved the HK416 to be more reliable?
According to an article by 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 (automatically goes into the forward/unlocked postion).
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.
http://img514.imageshack.us/img514/46...(Picture taken from HK 2007 NDIA brochure.)
AK
The AK bolt head (like some other designs) is not forced to rotate earlier than needed to lock.
http://img171.imageshack.us/img171/84...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):
http://img530.imageshack.us/img530/27...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 on it's forward and rearward move.
http://img257.imageshack.us/img257/17...And there I you can find the following answer:
Sent: Friday, March 27, 2009
9:21AM
Jason,
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:
http://img512.imageshack.us/img512/33...http://img512.imageshack.us/img512/86...http://img512.imageshack.us/img512/48...May 2009
POF-USA Roller Cam
http://www.pof-usa.com/slideshow/roll... Category:
Science & Technology 
Tue Nov 24, 2009 11:47 am
