quote:

Originally posted by KSGM:
It's not about preventing cam pin wear though.

Is that you Gene?

quote:

Originally posted by jljones:

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.

100& Agree...Don't sweat it and enjoy your rifle.

IMO adding a steel insert to that area just adds weight, cost, complexity, and additional failure points to the design in an effort to fix a problem that isn't a problem. By the time an upper receiver wears out, you're looking at replacing a lot more expensive parts anyway.

If OCD compels someone to just absolutely need a steel bearing surface in that area, then there are options out there, but you'll spend way more than the feature is worth in practice.

The idea of replaceable inserts on the MCX always cracked me up anyway, because knowing Sig, by the time you wear one out and need a replacement they'll have discontinued the platform (or be on Gen 17) and you won't be able to get the parts anyway.

I am confident no one is really sweating the wear on the upper receiver. This has been a discussion about what that wear potentially means, if it's an indicator of a real flaw, and why such a small part of the industry has chosen to consider it, while the larger part of the industry is content to create a bunch of truly superficial stuff.

Also, this is a weapon nerd conversation, more than a practical one. I consider myself a practical shooter, or "end user"; in the context of my practical application of an AR15, I am not worried about this galling phenomenon. However, I am also intrigued by contemporary weapons from the engineering and design perspective.

I've said it a few times already, in different ways: the concern isn't receiver wear, or cam pin wear; the condition that causes the wear is the intriguing factor.

I agree with 92fstech, partially. I think that, if a steel insert is truly an improvement, the minimal weight and cost associated with it's implementation is a non-issue. Complexity is another story; one I have certainly considered.

As with most other speculative AR15 improvement theories, it would take a side-by-side evaluation over tens of thousands of rounds to prove the worth of a steel bearing surface. Does a traditionally built gun experience a failure that could some way be traced to that contact patch before a gun with an insert? Does a gun with a steel insert exhibit less wear in critical places, like the bolt, than a gun without the steel? Does the steel piece itself cause problems at a rate that, when compared to the maintenance cycle of the traditional gun, make it impractical? It'd be an interesting test, that no one can really afford to do.

I had the thought to email both Chris Bartocci of Small Arms Solutions and Chad Albrecht of School of the American Rifle. I think I got an email out to Bartocci, but Albrecht seems more inaccessible.

Remember this is academic! So let's not get distracted by the "shut up and shoot" thing. I am going to shoot a bit today, and I promise I won't spend a fraction of a second worrying about my cam pin or upper receiver.

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

quote:

Originally posted by 92fstech:
IMO adding a steel insert to that area just adds weight, cost, complexity, and additional failure points to the design in an effort to fix a problem that isn't a problem. By the time an upper receiver wears out, you're looking at replacing a lot more expensive parts anyway.

If OCD compels someone to just absolutely need a steel bearing surface in that area, then there are options out there, but you'll spend way more than the feature is worth in practice.

The idea of replaceable inserts on the MCX always cracked me up anyway, because knowing Sig, by the time you wear one out and need a replacement they'll have discontinued the platform (or be on Gen 17) and you won't be able to get the parts anyway.

The weight is negligible. I agree re: complexity as it relates to a potential failure point. The options out there if one wants it are the Colt 694x series (I like these even if antiquated in fore end length), MCX (yuck), and Haenel cr223 (not viable due to lack of spare parts).


Re: the MCX, I agree. I’m done being an Exeter beta tester. I was all set to get a Spear and Spear Lt, then the handguard shift issues came to light.

The wear is typically a self limiting phenomenon. It gets to a certain point then gets no worse. Upper receivers are cheap, so stock up on spares.

Again, this is an academic discussion. A steel cam pin bearing plate is an interesting addition when done correctly. It certainly doesn’t need it. Geissele added one on their 6 ARC submission that runs the entire length of the left side and it’s riveted in place.

That is normal wear. It is caused by the opening stroke during unlocking. It is not an issue. Internal piston, that wear stops at a point. Now external piston operated AR’s are different story. They have a much harsher opening stroke. The cam pin guard is to prevent that damage from getting more extreme. Check out my video on Colt external piston history. I go over this. Internal piston rifles do not required any form of guard. The cam pin has to engage these surfaces to lock and unlock.
-Chris Bartocci

Well, it still seems logical to me to have a steel insert if the designers know that the steel cam pin has to touch that area.

You know what? The more I think on it, the more it doesn't make sense to me. I felt like JoshNC was onto something, when he hypothesized it occurs on the feeding stroke, due to the bolt running into the back of the round being fed. Mr. Bartocci says recoil stroke. I don't understand why the bolt (and therefore camp pin) would rotate anywhere other than TDC during the recoil stroke, in a DI gun. In a DI gun, the expanding gas is not only pushing the carrier rearward, but also holding the bolt forward, right? If that is right, then the bolt would be incapable of moving rearward at all until the carrier pulls it rearward because the cam pin is lodged in the front of the cam path, and therefore at TDC. It seems it would have no reason to contact the rear of that pocket, when the BCG is moving rearward.

Actually, even on the feeding stroke, the moment the bolt face makes contact with the round being fed is so far aft of the cam pin pocket, that it couldn't hardly be causing it either.

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

quote:

Originally posted by KSGM:
That is normal wear. It is caused by the opening stroke during unlocking. It is not an issue. Internal piston, that wear stops at a point. Now external piston operated AR’s are different story. They have a much harsher opening stroke. The cam pin guard is to prevent that damage from getting more extreme. Check out my video on Colt external piston history. I go over this. Internal piston rifles do not required any form of guard. The cam pin has to engage these surfaces to lock and unlock.
-Chris Bartocci

Well, it still seems logical to me to have a steel insert if the designers know that the steel cam pin has to touch that area.

You know what? The more I think on it, the more it doesn't make sense to me. I felt like JoshNC was onto something, when he hypothesized it occurs on the feeding stroke, due to the bolt running into the back of the round being fed. Mr. Bartocci says recoil stroke. I don't understand why the bolt (and therefore camp pin) would rotate anywhere other than TDC during the recoil stroke, in a DI gun. In a DI gun, the expanding gas is not only pushing the carrier rearward, but also holding the bolt forward, right? If that is right, then the bolt would be incapable of moving rearward at all until the carrier pulls it rearward because the cam pin is lodged in the front of the cam path, and therefore at TDC. It seems it would have no reason to contact the rear of that pocket, when the BCG is moving rearward.

Actually, even on the feeding stroke, the moment the bolt face makes contact with the round being fed is so far aft of the cam pin pocket, that it couldn't hardly be causing it either.

By the nature of the design, it’s occurring on forward and rearward stroke.

I'd say the nature of the design has the cam pin at TDC at every point in the length of travel, except for exactly where that pocket is. Hence the location and dimensions of the pocket. Some shortcoming of the design has the cam pin offset from TDC, at that point just behind the pocket (in many rifles).

It would be neat to see high-speed camera footage of that area, via a cut-away in the receiver. That could tell us if it's more prone to occur in the forward or rearward stroke, and maybe give an indication of it's cause.

The fact that Colt, and other manufacturers, address it with the steel bearing surface, and not an alternative cam pin or altered interior receiver dimensions, tells me that it's important to them to keep the cam pin at TDC, because that is indeed, the intent (or nature) of the design.

I suspect that the phenomenon is more prevalent in built guns, or guns by manufacturers with looser tolerance requirements. As an example, a friend's factory KAC upper only has an ever-so-slight contact patch. He hasn't used it with a silencer though; so we'll see if it changes down the road, if/when he does put a can on it. As it is now, it's almost just a polished spot, with almost no deformation of the inside of the receiver.

quote:

Originally posted by KSGM:
I'd say the nature of the design has the cam pin at TDC at every point in the length of travel, except for exactly where that pocket is. Hence the location and dimensions of the pocket. Some shortcoming of the design has the cam pin offset from TDC, at that point just behind the pocket (in many rifles).

It would be neat to see high-speed camera footage of that area, via a cut-away in the receiver. That could tell us if it's more prone to occur in the forward or rearward stroke, and maybe give an indication of it's cause.

The fact that Colt, and other manufacturers, address it with the steel bearing surface, and not an alternative cam pin or altered interior receiver dimensions, tells me that it's important to them to keep the cam pin at TDC, because that is indeed, the intent (or nature) of the design.

I suspect that the phenomenon is more prevalent in built guns, or guns by manufacturers with looser tolerance requirements. As an example, a friend's factory KAC upper only has an ever-so-slight contact patch. He hasn't used it with a silencer though; so we'll see if it changes down the road, if/when he does put a can on it. As it is now, it's almost just a polished spot, with almost no deformation of the inside of the receiver.

Much as I love to nerd out on this topic, it’s really something that isn’t a real problem. I like the idea of future proof guns and having an upper that is a “forever” upper is appealing. As it stands now, I stocked up on new Colt m4 uppers for replacement purposes if necessary in the future.

Right along, this has been about the condition(s) that potentially cause the wear patch, and what they might mean. It has never been about the lifespan of an upper or cam pin. Some other, more critical component is going to fail (probably multiple times) before the upper will need replaced. I am speculating whether or not the condition that contributes to the wear behind the pocket also contributes to premature failure of more critical components. By the time you deem it necessary to replace your upper, maybe you've been through two bolts; maybe both bolt breakages would have been prevented had the condition that also caused the wear patch been eliminated. That does make sense, doesn't it?