Scratches and marks in AR chambers..... What is excessive and what is OK?

[mdewitt71]

So I recently got a bore scope the Lymans version.... BORECAM.
I decided to scope my two newest Grendel uppers first and found issues with both.
At this point I am just not sure how significant it is.

The rifling and muzzle crown seems fine in both, nothing to note but, both chambers have marks and nicks in them.
So far this is the first two of my rifles so, I am not sure if I will find "flaws" in every rifle I own yet....

12" in PSA Upper ($349 on sale with less than 50 rounds thru it).






The first time I shot it, the brass was marked up a lil....
I have polished it with some Flitz and will try it out tomorrow.

18" Satern SS Fluted barrel (about 350 round thru it now).
Rifle shoots great (Sub MOA often) and no marks on brass what so ever.


I will look at more rifles tonight and see what I find....
I know 80% of most shooters don't have access to scopes or care to go the distance to see what theirs really look like.

[JASmith]

The obvious scratches in the bottom image suggest careless tool handling by the person who cut the chamber.

I hope, however, that one of our members who has experience in metallurgy or inspection can give us a read on the first two images. I have only a lttle experience in this area, but they suggest that the barrel steel itself has cracks and inclusions. That could be a safety issue.

[mdewitt71]

The obvious scratches in the bottom omage suggest careless tool handling by the person who cut the chamber.

I hope, however, that one of our members who has experience in metallurgy or inspection can give us a read on the first two images. I have only a lttle experience in this area, but they suggest that the barrel steel itself has cracks and inclusions. That could be a safety issue.

I see no cracks...
I have seen fire cracking throughout the rifling and chambers of a lot of older barrels in the past but, theses are scratches and nicks in the chamber neck & throat.

[Kswhitetails]

I recently attended a precision rifle class where this very topic was discussed. The instructor was a long time precision rifle builder and gunsmith that worked for years at (among other places) Surgeon rifles (pre-merger). He stated that the new bore cams are affordable, and as such allow non-versed rank amateurs to view the other side of precision chambers. The problem as he saw it, was that folks just don't know what to look for, what to ignore.

He said that he would rather a new bore scope owner observe, shoot, and observe more. Don't think that imperfections in the machining process will likely have any real effect on accuracy. Small tooling marks and gouges are routine, even on high end chambered barrels. The trick is, to keep shooting as you always have, observing how the rifle performs, not the condition of the chamber and bore. Only when you notice a degredation of repeatable accuracy should you begin to employ these tools; and only when you have a good idea what it is you're looking for. Education materials are - for now, slim concerning these tools.

This was in the final briefing for the class, and was more in passing than most anything else. He was impressed with the new generation of the Lyman bore scope, and said that it was almost too good, at too low a price. Loose quote: "Now, everyone that has one of these new higher resolution gadgets will get swamped in worrying about something that they would have been blissfully ignorant of before." His point was that they are useful, for knowledgeable gunsmiths, and for general inspection. The truest test of a chamber isn't it's machined perfection, but how well it puts repeated hits downrange.

He did say that you will know what to look for if you ever see a worn out barrel through one. One was all it would take. Look at a known "shot-out" barrel, and you will see the obvious problems a bore scope will help you keep an eye out for.

Perhaps it would help to compare these observations with a barrel that has several hundred or thousand rounds through it?

Oh, (take this with a grain of salt, for my memory may be off here, but) he also said that every barrel treatment looks different under magnification. Chrome lining in the throats starts to show cracks long before it becomes an accuracy issue, whereas in a nitrided barrel, cracking is indicative of fatigue that should be more closely observed. Stainless and CROMO steel untreated barrels were best to cut new teeth on because they are typically more honest indicators of barrel life than the treatments, which change surface hardness in sometimes strange and irregular ways.

[Frontier Gear]

This reminds me of when IR cameras started to become affordable. Back when they were $30,000 only business who were serious about it would buy them and they would send someone to the week long class on how to use them. Once the price dropped, people (non-businesses) and smaller business would buy them but not invest the time or money in the week long class. The next thing you know, there are a bunch of false readings being reported because people don't know how to configure the camera or understand things such as emissivity or reflective materials. I'm not an expert at reading bore scopes, but my gut is telling me that if they shoot well, not to worry about it.

[JASmith]

I see no cracks...
I have seen fire cracking throughout the rifling and chambers of a lot of older barrels in the past but, theses are scratches and nicks in the chamber neck & throat.

The features are likely not fire cracking. I included "cracks" to help widen the scope of investigation.

The images do, however, suggest that there are voids in the steel that were uncovered as it was cut. The only one I can positively attribute to tooling is the last one that shows an obvious scratch.

That is why I hope that someone with experience in inspecting machined parts or metallurgy can give us a reading.

If the features are indeed voids or inclusions, then there could be a safety issue.

[Bearcatrp]

Have read many times to never look inside the barrel. My 10T and 10 FCP-SR look similar. Even my faxon match barrel looks a little like that. One of the reasons to foul the barrel to fill the imperfections. I made the mistake of cleaning my 10T before a Easter egg shoot @100 yards and missed both shots. Previous year hit on 1st shot. Just go shoot and enjoy your rifle.

[mdewitt71]

I took my scope to work and looked at several DOD systems this morning..... Man, I'm not worried at all about mine haha.

[LRRPF52]

The main thing you might want to look at is the gas port.

Sometimes, gas ports are cut so roughly that a large burr is left over, which will break off a large chunk of the gas port aperture in the bore, leading to early gas port erosion.

Different manufacturers have their own ways of cutting gas ports to reduce the possibility of this happening.

[Sticks]

I'd say if your brass is getting marked up then it's worth taking a look to see why, especially if this is a new issue with a chamber (currently going through that my own self).

Otherwise it's just another toy to have on the shelf. Better use would be to look at a crown without having to remove a muzzle device.

[mdewitt71]

I'd say if your brass is getting marked up then it's worth taking a look to see why, especially if this is a new issue with a chamber (currently going through that my own self).

Otherwise it's just another toy to have on the shelf. Better use would be to look at a crown without having to remove a muzzle device.

So, I polished the chamber from LRRPF52's instructions with Flitz and the new chamber brush.
I was hoping to get out to the range today and do some more load testing but, we are under severe weather warning......
I might try to rip a few round out in my backyard pistol range just to check the brass though.

[keystone183]

Slight sidetrack..........if i may op.........

While i was reading and researching about chamber polishing, i came across a few people with concerns of overpolishing a chamber and reducing the case's ability to "grip" when under pressure, creating a potentially unsafe situation. Anybody think those concerns are valid?

[Kswhitetails]

They are. But if a person is careful to do things within reason, highly unlikely.

Keystone, if I may: I am no gunsmith, and these are just my thoughts as a shade tree mechanic that loves to work on my guns.

It is possible to polish a chamber with such a fine compound for so long that it reduces the surface friction coefficient by making the surface too smooth, which increases bolt thrust. It is also possible, and more likely in my estimation, to polish with a more course compound so much that you begin to expand the chamber dimensions. Either of these are unwanted circumstances.

The finer compounds cut slower, and require more RPM to get their job done. So with a finer compound, it is easier to control how much you're changing the chamber dimensionally. If you turn slowly, for less time, by it's nature finer compounds make an excellent chamber finish choice. Just don't go fast for extended periods - I.E. greater than two minutes at a time.

The course compounds cut quickly, but leave lots of surface friction so that isn't the issue. The problem here is that you can't be too careful. You want to get the surface "finished" without changing the chamber dimensions by removing too much material. The more course the compound the quicker this happens.

If it were me, I'd opt for some 1000+ grit. If I were way out there away from a source of that, and all I had was some 600 grit, I'd run a mop or brush or plug for 20 seconds at low speed - think 2-300 rpm, usually low gear on the new cordless drills. Then, I'd clean the chamber, drop a round in, feel for proper chambering, and run another 20 seconds if I felt it was needed. If I didn't know, or wasn't sure, I'd assemble and test fire before I continued. You want the brass to look good after firing (free of obvious scratches - some small minor scratches are ok, and will naturally polish out with use), and the rounds need to feed well and extract well. If these conditions are met in my test, then a tear down, and final assembly is in order. If more work is needed, a tear down, rinse and repeat.

I am no gunsmith, and these are just my thoughts as a shade tree mechanic that loves to work on my guns.

This isn't rocket science, and can easily be done at home with a modicum of care and patience. These are the keys to this work, don't do anything fast or take shortcuts. Additionally, the work done this way is most rewarding. Good luck!

[mdewitt71]

So I went thru a dozen DOD rifles yesterday, all of them had issues in the barrels some looked down right ugly... but, they still shot wekk and no brass markings out of the usual. Even got my hands on some new FN M4A1s we just so happened to issue yesterday.


I didnt take pics because my SD card was at home but, none were perfect, even out of the plastic.... Mil Spec baby haha.

I went thru 5 of my own rifles today.... I can say my Faxon Match-Grade Fluted 20 inch Wylde is a very nice looking rifle compared to many others.

[mdewitt71]

My Satern 6.5G barrel shoots well and does not leave marks on the brass but, inside the barrel with only about 400 rounds thru it, it was quite ugly.




My $349 PSA upper is not as bad as I originally thought....
The rifling was almost flawless despite the nics and scratch in the chamber neck and throat from the OP pics.
I am content with the way it shoots for sure.