Wear pattern developing on Colt upper

[ksss]

I discovered an alarming wear pattern on my Colt upper receiver. Inside the receiver adjacent to the halfmoon cutout it appears that my bolt cam pin is wearing into the receiver. The cut is caused by the cam pin I am sure. Why it is cutting into the receiver I am not sure. I checked the gas rings on the bolt, they are all three present and still provide resistance when operated by hand. Anyone have an idea? The rifle currently functions well. However, it is a personally owned duty rifle and I want to maintain its, so far, excellent reliability record. I don't know excactly how many rounds I've put through it but I bought new in 1987 and have shot it extensively. The rifle is a Colt preban A2 Hbar with a 20" Bushmaster fluted barrel.

[carbonblack]

What do you mean by "alarming wear pattern"? Is the cam pin battering the receiver? Have you ever replaced the buffer spring? Does the headspace check out Ok? The hole in the cam pin that the firing pin runs through is not wallowed out? Is the firing pin Ok? More details, some very knowledgeable folks hang around this board, you might want to post this is non-glock firearms.

[DJ Niner]

I've seen this type of mark inside a few AR uppers, if it's located between the half-moon cut and the rear face of the barrel, along the edge of the charging handle track. My current pre-ban Colt also shows slight wear in this area, although none of my Bushies do.

If this is the area of concern, first, I'd check the bolt carrier for burrs or sharp edges. Holding the carrier with the gas tube key on top, and the bolt pointing away from you, look along the left side of the carrier key where the slightly raised "pad" is; these pads (top and bottom) are the only parts of the carrier that are supposed to touch the upper receiver during use. Check the leading edge of this pad by rubbing your finger across it; if it's sharp, it should be gently rounded with a needle file or stone, and then all shaving/filing/abrasive grit carefully cleaned-off. Do the same check on side edges of the carrier key, too.

If it IS the cam pin, this would be somewhat unusual, but should be easy to check. Smear a light coat of colored candle wax (lipstick actually works better) on the outer corners of the cam pin in the assembled bolt carrier and work it in and out a few times by hand, then check for matching smears inside the receiver. If none are found, redo the test by firing with live ammo (just a few rounds). If there is still no evidence of color, then I'd say the cam pin is not the culprit.

Was the Bushy barrel replaced in the old/former Colt upper, or is a complete new Bushmaster upper unit? If it's a mix-n-match deal, then I suppose the barrel could be too far (or not far enough) into the upper, causing the bolt carrier to be too far rearward (or not far enough) during unlocking, causing the cam pin to smack the upper's sidewall (in the "too far in" case, it would actually be BEHIND the half-moon cut, not in front of it); but again, this would be unusual. Same thing could happen if the cam pin is a replacement from an unknown manufacturer; could be out-of-spec (head too large, corners not rounded enough, etc.).

[ksss]

Yes the pin is battering the receiver. It is cutting into the receiver adjacent to the cutout. The cut matches the top of the bolt cam pin. Buffer spring has never been replaced. I thought about the headspace issue. I contacted one guy that should know (Colt armorer) and was told that the headspace varies in the Colt AR's and can't be gauged reliably. Doesn't make a lot sense to me as I know that while in the Corps our rifles had headspacing checked. I have never seen a rifle develop this cut in the receiver and I have seen some vary high mileage M-16's. There is nothing wallowed out anywhere. The firing pin is in good shape. I have been over the rifle in great detail. You may have something with the bufferspring. Although I have never known one to wear out, I guess it is possible. I have a feeling that the bolt is coming back too fast before the bolt can unlock which could have something to do with the bufferspring.

Great suggestions on confirming if is the cam pin. I will try it and yes the upper is still a Colt.

[DJ Niner]

If you look at the rear face of the barrel extension, and the inside rear face of the upper receiver right next to the barrel extension, are they nearly level, or is the extension sticking out or sunk-in in relation to the inside rear face of the upper?

Personally, I don't believe the unlocking/travel speed of the bolt could have anything to so with this problem; the bolt has to be completely rotated (unlocked) before it can withdraw from the extension, and the movement of the cam pin (thus the name) is what makes it unlock. Whether it does it fast or slow, it's gotta be out of the way (under the key) before the bolt withdraws from the extension; so the bolt carrier's position relative to the upper is really the only way it could be forced out-of-position, the way I understand it.

How tall is the cam pin, measurd from the top of the firing pin hole to the top of the pin's head? If it's too tall, then THAT could cause impact with the half-mood cut during unlocking...

[DJ Niner]

Measurements on a mil-spec cam pin I have in my spares kit are:

Top of FP hole to top of pin -- .525"
Width of head -- .32"
Length of head -- .40"

[DJ Niner]

Originally posted by ksss
Yes the pin is battering the receiver. It is cutting into the receiver adjacent to the cutout. The cut matches the top of the bolt cam pin. When you say "adjacent" to the cutout, where exactly do you mean? Toward the front of the rifle, the rear, or out to the side of the cutout?

[jsanti13]

I have that same problem on my preban bushmaster.

[ksss]

DJ,
I appreciate your help. This cut into the receiver is between the cutout and the rear of the receiver. I tried the lip stick trick on the cam pin and it leaves lipstick in the cut. If I understand correctly your asking if (I have the upper receiver with the barrel down) top of the locking lugs are even, up or down from the upper receiver at the point (near the upper take-down pin)that the barrel extention (which I think to be the locking lugs) comes in contact with the receiver. If I understand correctly then the top of lugs are fractionally lower than the receiver. Also the bolt cam pin is an original as is everything except the barrel. Is there any truth to the headspace being difficult to gauge? I am wondering if since the barrel was replaced if its not a headspace issue which is causing the bolt not to "seat" in the receiver/barrel extension as it is supposed to. Again I appreciate your help.

[DJ Niner]

Okay, let's take this one step at a time...

I don't think I was clear enough on my description above; I'll try again in a moment. But first, I want to say that even if your parts were ALL original, you could STILL have a bad/out-of-spec part, or even more than one. Guns are mechanical objects, made by fallible humans; bad parts happen. I've seen handguns stamped with the wrong caliber from the factory; a bolt-action rifle with a rifled, but not chambered, barrel (and a test target, too!); and at least one semiautomatic .22 rimfire with no rifling at all. It happens.

First possibility: The cam pin or cam pin cutout is the wrong size. If the cam pin is too long/tall, it hits the cutout wall; same thing happens if the cam pin is the RIGHT size, but the cutout is too small for it to move around in. Cam pin dimensions are in one of my other posts, above; check'em and see what you get. Measuring the cutout is a bit trickier; not only depth/height, but location front-to-back within the upper receiver would be critical, and I'm not sure of the allowable variances for these dimensions. Best thing to do here would probably be to get (borrow?) another AR, and do a bunch of measurements with a good machinist's scale/ruler, such as: how far in from the rear of the upper does the cam pin cutout begin? And end? How wide is it? Tall? Etc. Just measure from the same point on each upper and see what you find. Use another Colt to compare if possible; other manufacturers may use other dimensions.

Second possibility: The barrel was not installed correctly, or the installer changed some critical dimension on your upper during installation. Let's say your upper receiver looked a little rough at the front of the upper's barrel nut threads when the installer took the old barrel off. So, he decides to "clean up" or true-up the end of the upper with a file, shaving off some metal. Now, when he seats and torques the new barrel into place, the barrel collar (which seats agains this area on the front of the upper) allows the barrel to slide deeper into the upper than normal. Folks with only a basic understanding of the AR system would say "So what? The bolt locks into the barrel extension, not the upper receiver!" This is true, but if the barrel/extension is too far into the upper, then the bolt carrier will be farther to the rear when the bolt locks and unlocks, which could cause (you guessed it!) the cam pin to smack the receiver wall behind the cutout. So, what I was referring to in the above post was the rear of the barrel extension (the rearmost part of the toothy area where the bolt goes in and locks), and the flat area of the upper AROUND this barrel extension, when viewed from inside the upper receiver. On your Colt, there should be two little "mini feed ramps" cut into the bottom of the barrel extension, where the rounds slide into the chamber as they feed from the left and right side of the mag. On mine, these cuts actually extend into the aluminum of the upper receiver slightly. THIS is the area of the upper I want you to check; where the barrel extension pokes though into the upper receiver. Looking (from the inside/bottom) at the surrounding upper, does the barrel extension look like it's extending too far in, so it is raised in relation to the part of the upper near the ramps? Or is it sunken-in, looking like it wasn't slid far enough into the receiver? Or is it just about even with the receiver metal?

There could be other areas that affect the location/depth of the cam pin movement, but I'd say these are the most probable two items which could cause your problem.

[ksss]

DJ niner

I understand what you are saying about the ramps cut in to the receiver slightly. Mine however does not have that. Interestingly I can see where the aluminum is very slightly worn in front of the two ramps from the rounds entering the chamber I think. The cut starts on the right side (upper receiver upside down, ejection port facing you) of the cutout in receiver it is deepest closest to the cutout and shallows as it extends toward the rear of the receiver. It is about .25 of an inch in length and maybe .5mm in depth at its most severe point. The barrel extension in relation to receiver is slightly sunken. I will get a hold of a micrometer and mic the cam pin and the cut out. If it helps I put lip stick on the face of the bolt carrier and inserted into the receiver and ran it forward. I have contact between the carrier and the barrel extension. Do you have any information on the headspacing spec on these rifles?
Thanks

[DJ Niner]

As I understand it, headspace on an AR pattern rifle/carbine is measured completely inside the chamber and barrel extension area; from the face of the bolt (where the base of the cartridge rests) to the shoulder of the chamber (where the shoulder of the cartridge rests). To check the headspace, the chamber is cleaned and degreased (as is the headspace gauge), the gauge is dropped into the chamber, and the bolt carrier is pushed forward by hand from the rear of the upper (upper receiver group is detached or pivoted open for this check) until the bolt is closed or it stops. If the rear of the bolt carrier is flush with, or below the rear surface of the upper receiver (bolt has closed and locked completely) the weapon has excessive headspace. If this is the case, the bolt is replaced with a new one, and the headspace is checked again. If it passes (bolt fails to close and lock on the gauge), rifle is okay with the new bolt. If it fails again, the barrel assembly is replaced, as the headspace within the barrel assembly is set when the barrel extension is installed during barrel construction.

Because of the way the bolt locks into the barrel/extension, the headspace really cannot have anything to do with your problem, as far as I can see. Movement of the cam pin is controlled entirely by the barrel's breech stopping the forward motion of the bolt, and the remaining forward motion of the bolt carrier cams (rotates) the bolt to the locked position, forcing the cam pin out to the side (into the cutout) and stopping as the carrier strikes the rear of the barrel extension. When locked, the cam pin overhangs the left side of the carrier, and should be aligned with the cutout in the upper which was provided for this reason. If the pin is hitting the receiver behind (rearward of) the cutout, then the bolt AND barrel assembly must be too far to the rear in the upper receiver during unlocking, IF it's a problem related to the barrel/bolt carrier position. This is why I was asking about the position of the barrel extension in relation to the inside front face of the upper receiver; if it's too far to the rear, then it's forcing the bolt carrier to be too far back during unlocking, causing the cam pin to strike the wall behind the normal cutout area.

[ksss]

DJ Niner,
I appreciate your help on this. I am going to take it to another armorer and see if I can have the barrel backed out and again try the lip stick and see if it is still hitting. I will also have the cam pin miked. Thanks again for your help.

[DJ Niner]

Let me know what you find out; this has really got me curious as to the exact cause...

Good luck!

[Noban]

ksss - I had the same concern with my post ban Bushy and called the company armorer. He told me the "problem" is common and that it is caused by some of the upper receivers being out of spec, just slightly. Essentially, too much metal in the wear area. He told me that once the metal wears down to spec, the wear will cease. Right as rain. After about 200 rounds, no more wear, just a shiny spot. I now have about 6,000 rounds through the upper with no further problems.

I'd just shoot it.

[gasrings]

It happens. Not worried - it wears a bit and stops.

This is a Colt M16 upper... pushing 40 years old. Still rockin-n-rollin.

http://img.photobucket.com/albums/v151/gregw45/campinwear.jpg