Both of the below effects have always been noted with this 5.56 AR (Colt LE6920) that now has about 20,000 rounds total and a bit over 2000 rounds on the current barrel. I have owned the carbine for 16 years and it was acquired new. I have other ARs of my own and routinely inspect my agency’s several weapons, and have not seen similar effects with any other gun.

The current barrel and gas block were acquired new from Brownells, but made no difference in the effects. Prior to replacing the barrel and gas block along with a new Colt gas tube, I replaced the gas tube with an original factory part with the original barrel once.

This is my primary training gun. Ammunition has always been factory new, and virtually all has been Federal or Lake City FMJ, either 223 Remington, or M193 or M855 5.56.

Except as pictured, I can’t find any damage or unusual wear on any other parts, including the bolt carrier or firing pin.

The finish on the face of the buffer is worn off by contact with the bolt carrier. It has happened with more than one buffer and starts occurring within a short time after a new buffer is installed.



The firing pin retaining pins become bent in use. Again, it occurs within a short time, and I have replaced the pin many times. I’ve tried the old solid style pin a couple of times. One shows similar battering and bending (although not as much), and the slotted end of one broke off.



Any ideas why this one particular carbine would be susceptible to the effects seen?

This particular rifle has had much more use than my other guns or any of my agency’s guns. Even though the effects seem to appear quickly after a new part is installed, is it just normal wear?

Is it a 16" barrel with a carbine length gas system? I believe those rifles cycle harder than 14.5" barrels with carbine length gas systems and 20" barrels with standard gas systems. The bullet is in the barrel 1.5" longer past the gas port than it was designed for.

The mid-length gas system solves this, if I remember correctly. For this reason, I went with a 16" BCM mid-length.

Tony.

quote:

Originally posted by benny6:
Is it a 16" barrel with a carbine length gas system?

Yes, but so are all the other ARs I own and inspect that don't exhibit the same effects. Several of those guns are identical model Colts.

Hmm. Interesting. Are all the bolts full-auto BCG's, or is there anything different with the bolts? I do remember that full-auto BCG's have more mass and cycle slower than the non-full autos from 20 years ago. I'm assuming buffer springs are replaced regularly too?

Tony.

quote:

Originally posted by benny6:
Hmm. Interesting. Are all the bolts full-auto BCG's ...?

Yes.

I admit, though, that the buffer springs should probably be replaced more often: what is recommended? (The wear effect nevertheless still started shortly after I acquired the gun.)

Have you replaced the BCG? Does it have the older style partially unshrouded firing pin, or is it fully shrouded?

The buffer turns slightly during cycling, the small amount of motion causes wear between the faces. A rough face on the rear of the carrier would cause this to happen faster. More typically a burr on the carrier results in a scratch or gouge forming from being dragged across the buffer face when splitting the receivers, or a radial mark forming from wear.

I wouldn't consider this troublesome unless the buffer's face is forming a groove and it's deeper than finish wear.

The firing pin retaining pin is most likely due to a tolerance issue. The hole for the retaining pin can be slightly off, the bolt dimension could be slightly short allowing the firing pin more forward travel, or it could be happening from excessive gap at the tail of the bolt where it fits into the carrier allowing too much gas past it and blowing the firing pin to the rear with excessive force.

Has firing pin protrusion been checked?

Has the FPRP done this since new, or is it a more recent occurrence? It happens quickly, but has it always done this?

Can you tell if the FPRP is bent forward or back?

Do you clean the tail of the bolt? If so, do you scrape the carbon, or use a metal bristle brush?

Look up School of the American Rifle on YouTube. Might be worth contacting him to see what he thinks and he may even offer to take a look at it.

Definitely odd where you're saying it does it in a short amount of time after replacing parts.

Thanks for all that.
I don’t have a protrusion gauge, but by eye it appears the same as a couple of other firing pins I checked. I have never had any issues with the ammunition, and the primer indentation seems normal.
The bolt carrier has the fully enclosed rear, and the back end that contacts the buffer is smooth.
I keep the tail of the bolt pretty clean of carbon buildup. I usually use the mouth of an empty 223 brass case to scrape the area.
I’ll have to compare the position of the FPRP hole with another bolt carrier to see if there’s anything odd about it.
Now that I think about it, I don’t know when the FPRP bending started. It’s been a long time, but I don’t remember if it’s been a problem since the beginning. Possibly not, though, because it seems to me I noticed the buffer wear before the pin issue. I haven’t tried to determine the direction of the bend.

sigfreund,
next time you're in Denver bring the rifle to http://bowerstactical.com/. Very knowledgeable regarding diagnosing AR problems. They'll be able to tell you what's going on and have a deep inventory for anything that needs to be replaced. If you're able to go there, don't change anything on the rifle, bring it as is.

quote:

Originally posted by SIG228:
Look up School of the American Rifle on YouTube. Might be worth contacting him to see what he thinks and he may even offer to take a look at it.

Definitely odd where you're saying it does it in a short amount of time after replacing parts.

Chad at SOTAR is good people. His YouTube is under InstructorChad, I don't think it has the SOTAR name on it. Without attending one of his courses (I have not) soaking up knowledge is slow drips, with a lot of hours of videos to pour over hoping he might mention relative info if you go looking for a solution to this problem, for instance.

quote:

Originally posted by sigfreund:
Thanks for all that.
I don’t have a protrusion gauge, but by eye it appears the same as a couple of other firing pins I checked. I have never had any issues with the ammunition, and the primer indentation seems normal.
The bolt carrier has the fully enclosed rear, and the back end that contacts the buffer is smooth.
I keep the tail of the bolt pretty clean of carbon buildup. I usually use the mouth of an empty 223 brass case to scrape the area.
I’ll have to compare the position of the FPRP hole with another bolt carrier to see if there’s anything odd about it.
Now that I think about it, I don’t know when the FPRP bending started. It’s been a long time, but I don’t remember if it’s been a problem since the beginning. Possibly not, though, because it seems to me I noticed the buffer wear before the pin issue. I haven’t tried to determine the direction of the bend.

One of the bits of knowledge gleaned from Chad/SOTAR- scraping the carbon from the bolt does more harm than good, and cap open up the fit at the bolt tail. Not that brass is going to be a fast-wear tool to use, but he doesn't recommend anything more aggressive than nylon brushes to avoid the issue entirely.

I'd swap in a fresh buffer, BCG, and run the rifle through your normal paces and see if either issue remains.

If you've rebarreled and are still using the same bolt; was the bolt headspace checked to the new barrel? Not that I think that's your issue, but if a barrel is shot out replacing the bolt at the same time is fairly typical.

Thanks for all the comments.

Is the wear ring on the buffer the same inside and outside diameter as the back of the bolt? I'm wondering if the bolt carrier fits loose in the upper and is wiggling as it moves back, creating a larger wear mark on the buffer.

Tony.

Manufacturers have been known to use slightly different BCG that have different weights that otherwise look to be the same. A lighter BCG may be moving faster during recoil.

There can also be slight variations in the gas port diameter due to wear on tooling or lack of wear on a new tool. Again, this affects BCG velocity during recoil.

Bill

Bill

quote:

Originally posted by benny6:
Is the wear ring on the buffer the same inside and outside diameter as the back of the bolt?

Good question. The outside of the wear ring extends to the edges of the buffer face, but the inner diameter is virtually the same as the diameter of the inside of the bolt carrier. On the other hand I can wiggle the bolt carrier of the problem gun around somewhat more than that of a similar 6920. There is, however, no sign of excess wear on either the rails of the bolt carrier or on inside the receiver. I suppose I could take some measurements to see if there are any dimensional differences.

Things to think about.
The buffer wear seems to have no adverse effect on anything and FPRPs are cheap, so none of this is critical after nearly 20K rounds, but something I’ve long wondered about.

quote:

Originally posted by benny6:
Is it a 16" barrel with a carbine length gas system? I believe those rifles cycle harder than 14.5" barrels with carbine length gas systems and 20" barrels with standard gas systems. The bullet is in the barrel 1.5" longer past the gas port than it was designed for.

The mid-length gas system solves this, if I remember correctly. For this reason, I went with a 16" BCM mid-length.

Tony.

I believe the carbine length gas system was actually designed for the XM177s, which were originally a 10.5” barrel, IIRC. They kept the same length for the 14.5” M4s simply because 14.5” is still too short for the rifle length gas system, which was the only other option available without redesigning the gas system at the time. The extra 1.5” on 16” vs. 14.5” isn’t much worse. Both are pretty overgassed.

quote:

Originally posted by sigfreund:
I admit, though, that the buffer springs should probably be replaced more often: what is recommended? (The wear effect nevertheless still started shortly after I acquired the gun.)

Not sure about your other questions but according to the TM the springs are checked for free length.

quote:

Originally posted by mbinky:
[A]ccording to the TM the springs are checked for free length.

Thank you. After I started trying to find the information, I found the same in a manual provided in a Colt armorer course several years ago. It makes sense of course that spring life wouldn’t be based on round counts because that would be virtually impossible to monitor for military and even most LE users. I measured the action spring in my subject rifle and it’s 10 1/8 inches: within spec, but just barely.

Again, thanks to all who commented.

Weighted and compared BCG?
I am thinking the symptoms present themselves constantly, it got to be in the basic of components. Maybe this one just needs a stronger buffer spring?
Always enjoy learning from you, SigFreund.

Can you post pics of the bolt carrier to include the bottom?

quote:

Originally posted by JoshNC:
Can you post pics of the bolt carrier to include the bottom?

I will try to get to that this afternoon.
(Outdoors training session today; projected high 15°. )

What should I be looking for or highlighting? Just general wear?