..., in the AR15 upper receiver.

Why don't they just revise the inside dimensions of the AR15 upper receiver, so that the cam pin doesn't contact it? There have been a few "fixes" implemented by various companies:

Domed or otherwise dimensionally-modified cam pin heads.
Alternate cam pin paths in the bolt carrier (this is not done to specifically address this wear phenomenon, but it can make a difference).
Steel receiver inserts (typically replaceable).

Is there a reason we don't look at a worn-in receiver, and just make those dimensions the starting point? What adverse effect(s) could that have?

I understand that this wear happens to varying degrees across a wide range of circumstances, for more than a few reasons. I don't think it's correct to say that a properly built AR15 shouldn't experience it; though I could be wrong.

The cam pin has to contact that area. A better option is to add a removable steel bearing surface there ala Colt 694x, SigSauer mcx, Haenel cr223. Geissele added a steel bearing surface on their 6mm ARC submission.

Seems odd to imply that contact is by design. As if Eugene Stoner said "so this part of the aluminum receiver's interior will be galled by the steel cam pin during the recoil cycle; this is perfect".

What causes the pin to be out of alignment with the receiver's upper channel in the first place? The pin must be in the vertical position for the bolt to be released from the barrel extension. How would the bolt even be able to move rearward enough for the pin to gall the area aft of the pocket, if the pin wasn't in a position that would prohibit said galling?

quote:

Originally posted by KSGM:
Seems odd to imply that contact is by design. As if Eugene Stoner said "so this part of the aluminum receiver's interior will be galled by the steel cam pin during the recoil cycle; this is perfect".

But even on high round count guns, this isn’t something that takes the gun out of service.

Of course you're correct. My current rifle has taken quite the beating in the spot in question; I have zero concerns. I just always thought it odd that, with all the stuff the industry likes to mess with, it hasn't done more with that than it has.

Though it's ultimately of little-to-no consequence, it seems like something that could be refined. Lord knows the industry has all kinds of other needless stuff covered; how many options do we need for "cable management"?

quote:

Originally posted by KSGM:
Seems odd to imply that contact is by design. As if Eugene Stoner said "so this part of the aluminum receiver's interior will be galled by the steel cam pin during the recoil cycle; this is perfect".

What causes the pin to be out of alignment with the receiver's upper channel in the first place? The pin must be in the vertical position for the bolt to be released from the barrel extension. How would the bolt even be able to move rearward enough for the pin to gall the area aft of the pocket, if the pin wasn't in a position that would prohibit said galling?

The contact is just a mechanical fact. It’s the junction at which the cam pin begins its rotation, both on forward stroke and rearward stroke.

In the forward stroke, the bolt is pushing on the round as it strips it from the mag and feeds it into the chamber. The pressure imparted on the bolt by the cartridge loads the left side of the cam pin head upon the left side of the channel in the upper, which includes the edge at the rear of the cam pin pocket.

I have noticed by playing with quite a few carriers, bolts and pins that some have more travel than others in the cam path or raceway, if you will. I often wonder if the difference in travel makes a pin more or less likely to have issues and crack or sheer? Basically, if we take the production line of any company and apply stacked tolerances to these three components giving the cam more or less travel to contact, if that changes the impact velocity and therefore, durability? That may be complete nonsense of course.

The wear occurring on the feeding stroke is not something I had considered; I associated the wear with the presumably more violent recoil stroke. It makes more sense in the feeding process though, that the pin would be encouraged to lean away from TDC, when the bolt meets the resistance of the round it's stripping (as described by JoshNC). The steel inserts some companies employ make a lot of sense, when considering it this way; ensuring the bolt is perfectly aligned with the locking recesses is a good thing. The more your receiver is galled, the more unaligned the bolt could be when it meets the barrel extension.

All this still begs the question: why do some rifles experience the galling more than others? I always load my mags to a full thirty rounds; I suppose that could make an impact, as it makes the top round that much harder to strip.

Wear is definitely also occurring on the rearward stroke. External piston ARs tend to wear more than traditional inline gas ARs. It’s likely due to the fact that in a piston, instead of the bolt and carrier being pushed apart by gas expanding inside the carrier, the external piston AR has a rod pushing the carrier rearward and a more violent forcing of the cam pin through its cam path in the carrier. They also lack bolt gas rings, which allows the bolt to more easily move/rotate through the cam path and ha less resistance to being pushed onto the left side of the receiver’s cam pin bearing surface during forward stroke.

Interesting thought re: the top round being more difficult to strip. That could contribute.

Other factors likely include the quality of anodizing as it imparts hardness, how the rifle is gassed (eg an overgassed rifle will wear more).

I have a very early LMT 10.5” upper that has around 10k rounds on it, all fullauto and suppressed. The upper has missing anodizing from cam pin wear, but no galling behind the cam pin pocket. I have other uppers from Noveske, Colt, hk416 that exhibit wear and galling.

Personally I think the AR upper should employ a removable cam pin bearing plate and removable charging handle latch interface as is done on the MCX and Haenel cr223.

Hodge chamfers the transition into the cam pin pocket. I sold my hodge upper unfired, so I don’t know if that chamfering makes a difference.

I think the steel bearing plate is a better "fix" than chamfering. The chamfer allows the pin to depart from TDC before it really ought to; the steel keeps it where it ought to be.

quote:

Originally posted by KSGM:
I think the steel bearing plate is a better "fix" than chamfering. The chamfer allows the pin to depart from TDC before it really ought to; the steel keeps it where it ought to be.

Absolutely.

With all the "enhancements" the industry provides the AR15, to include no shortage of unique receiver sets, why on earth don't we see more uppers with the steel insert? The cam pin is already accepted as a wear item, so who cares if it gets a bit more wear due to steel-on-steel? It seems to me that ensuring the bolt's perfect alignment with the barrel extension ought to be a priority. Perhaps that minor misalignment that produces the galling is also responsible for some premature bolt failures?

It may be as simple as the usual suspects in these decisions, "that's different" and "that costs too much and isn't appreciated". Redesigning the carrier group and barrel group or making modifications is a niche in the market where others are cranking out $500 ARs and people say they're all the same. It may be other things but I've seen people shy away from something superior that doesn't fit their definition of the way it should be. Take that and be a manufacturer where introducing a change and modifying your production line is a multimillion-dollar decision versus replacing the cam pin and it becomes a hard sell.

It's not about preventing cam pin wear though.

Bottom line, the AR15 upper is viewed as an inexpensive, replaceable wear item. Until there is a mil solicitation calling for a steel cam pin bearing surface, no such changes will be implemented on mil rifles.

Stock up on spare replacement uppers.

This message has been edited. Last edited by: JoshNC,

It's not about the upper wear either. It's about the cam pin being where it ought to be, when it ought to be. The wear of the upper and cam pin are results of the bolt being partially rotated when it shouldn't be. It seems, to me, that the steel bearing surface guarantees proper bolt position through the entirety of the operating cycle. This would presumably mitigate bolt wear, and certainly mitigate cam pin shaft wear and cam pin channel wear, and eliminate upper receiver wear. The only accelerated wear would be of the cam pin head, which would result in a more frequent replacement cycle for an inexpensive part that's already a wear item.

I know my Aug has a sleeve that ensures the bolt is in the proper position at the right time. Might be a potential solution?

quote:

Originally posted by Anubismp:
I know my Aug has a sleeve that ensures the bolt is in the proper position at the right time. Might be a potential solution?

The AUG uses the sleeve because without it there is nothing to prevent early rotation of the bolt as it strips a round from the mag (pre-engagement). The AR has the head of the cam pin that rides in its channel in the upper and can only rotate once the cam pin head clears the cam pin pocket in the upper. The AR doesn’t need the bolt sleeve of the AUG. AR18 designs have the cam pin held by on if the bolt rails until such time that it clears the rail and the bolt can rotate. The Aug lacks a cam pin channel and bolt rails, so Steyr had to approach preventing pre-engagement in a novel way.

quote:

Originally posted by KSGM:
It's not about the upper wear either. It's about the cam pin being where it ought to be, when it ought to be. The wear of the upper and cam pin are results of the bolt being partially rotated when it shouldn't be. It seems, to me, that the steel bearing surface guarantees proper bolt position through the entirety of the operating cycle. This would presumably mitigate bolt wear, and certainly mitigate cam pin shaft wear and cam pin channel wear, and eliminate upper receiver wear. The only accelerated wear would be of the cam pin head, which would result in a more frequent replacement cycle for an inexpensive part that's already a wear item.

The steel cam pin bearing plate is a fun topic about which to nerd out. I’d love to see it implemented in contemporary traditional inline gas ARs. In practice, it’s just not that big of an issue or the mil would have requested a fix.

I just checked my two high mileage upper receivers and they both show wear around the bolt cam pin recess in the upper receiver. The wear patterns are different in both rifles, so I have to assume there is some tolerance stacking going on.

I also Googled the wear pattern and found it effects just about every non military manufacturer. What makes me wonder is in my 15 years in uniform, including three years as a unit armorer, I've never seen it on government property M16 or M4 rifles.