I was cleaning my AR tonight and noticed some pitting on the rear of my bolt.  

I just cleaned it a week ago and there was some carbon build-up then, but I removed it with CLP and my brush.

The pitting is located on the rear part of the bolt on the curved shoulder between the gas rings and where the bolt shrinks down to a same tube at the rear.  There is no pitting on the tube, it is isolated on the curved area between the gas rings and the rear tube.

What could cause this pitting.  At a glance it would look like the small bits of carbon that are left after you scrub.  Those small bits that you have to scrub forever to remove.

Only until I put a penlight to it did I realize that they were pits and not bumps...

Your knowledge is appreciated  =)

Thanks  Tim

Are you absolutely sure that it isn't uncleaned residue?  If so, yes, rust can and will appear on this area of the bolt.  After several cleanings, most of our bolts look like chrome in this area (those green scratch pads do a number on the phosphate coat) therefore, the metal is unprotected.  If carbon is allowed to stay in this area and you live in a high humidity climate, rust almost certainly follows in short order.

The good news is that as long as the pits aren't atrocious, your bolt is still servicable.  Just pay closer attention to the area because it will now be more succeptable than ever.


Saleen

There is no rust anywhere on the bolt/carrier.

I have never found rust anywhere on the weapon because I maintain it well...

I know it is not left-over carbon residue. I can positively recognize that foreign matter  from my many hours I spent with my platoon cleaning M-16's, 203's, SAW's and such...

It is definately pitting.

I had no misfires or jams at the range.  Nothing seemed amiss while at the range.  I did not notice this until taking the bolt out of the carrier and cleaning it.

What would cause this type of pitting ?

It could be you missed a cleaning or didn't scrub the carbon off one time and it started to bite into the metal. If it is very small I wouldn't worry to much. Just keep checking it when you clean your rifle to see if it is getting worse.

The pitting is not real deep and I am not concerned with the part failing but was trying to see if there was a way I could keep it from happening in the future.

Anyone else know what could have caused this ?


Thanks  Tim

It is caused by a lack of cleaning in a humid environment.  I have seen them rust before, but unless it is excessive, I wouldn't worry about it.  From your post it seems to be very small so don't sweat it.  To keep it from happening, always pull the bolt out and completely clean the rifle after firing.  Pay special attention to this section since you now know it is succeptable, and make sure you keep it oiled to protect it in the future between trips to the range.

That should solve your problem.


Saleen

I use a stainless steel bristle brush to scrub the shit out of that area.  I have never found pitting though.

Do you run a swamp cooler in your house?

Never seen rust anywhere on the rifle, ever.  I always clean the rifle as soon as I leave the range.

I dont use a steel brush on the bolt because the finish is still there.  I just keep scrubbing with the plasti brush until all the carbon deposits are completely gone.

I cleaned it all up last night and finished it off with a very light coat of CLP.

A plastic brush???? My GOD man, you have the patience of a saint if you managed to get that carbon build up off with a plastic brush!

Let's start with why this area is so much more difficult to clean.  You see, the AR's direct gas system dumps all of that hot gas directly on that onle section of the bolt after each and every round.  High pressure, heat, combustion by-products, that one tiny area really gets dumped on.  

As such, it takes alot of effort to get it absolutely clean.  If all of the carbon is not removed, rust can start to form.  I have been able to avoid this, and I live in a rather humid environment myself (S.E. Texas).  Here's the method I use, perhaps it will benefit you as well.

I start by using an old knife blade to scrape the larger chunks of carbon off.  Put a drop or two of solvent on as well to help break it all down.  After thoroughly scraping the area, take a synthetic scratch pad (3M green or maroon) add some more solvent, and wrap it around the part behind the rings.  Now spin the hell out of it back and forth until the section is absolutely clean.  After awhile, all of the blackened phosphate finish (and carbon) will be gone and the surface will look like chrome.  Don't worry, it also makes it easier to clean next time.  Finish off with a light coat of oil.

A plastic brush will never remove the metal's factory finish, which will most likely hide some of the carbon deposits that led to your rust.  My guess is that once you have a clean surface, the problem will go no further.  Good Luck.


Saleen  

Quoted: A plastic brush???? My GOD man, you have the patience of a saint if you managed to get that carbon build up off with a plastic brush!

you aren't kidding!

Even though i use the Stainless steel brush, it hasn't damaged any of the finish in that area.

May I suggest scrapeing the concave portion using a merry go round motion with the open neck of a spent.308 casing, and finishing with a used bronze bore brush.  No carbon and you get to keep your finish.  Very quick and effective.

edited to add - .223 brass will work just as well, you just need to make more passes as the .223 neck is smaller diameter.

Holy hell a plastic brush?!  I want this guy with me the next time I go camping.  That way if all of the lighters / matches fail, HE can rub the sticks together to start the fire.  When I was new to the land of AR I tried scrubbing that crap off, not about to happen in any time frame I thought suitable.  Whipped out the knife, off it comes.  (just be careful with the motion so you aren't gouging the metal.)

Mike

Heh   I can do that too !

The plastic brush is a left over habit from the Marines...  I have always used a plastic brush on my M-16A2 and will continue to on my AR.  I also use the firing pin to scrap the little bits of carbon off when I am almost finished cleaning.  Then one final scrub and I am done.

I use wire brushes on all my stainless steel pistols.  .357s, .44, Ruger Mk 10 .22LR.

But on my AR I use my USGI plastic brush


Can too much lube in that area cause pitting ?

Quoted: ...snip... I also use the firing pin to scrap the little bits of carbon off when I am almost finished cleaning.   ...snip...

Ouch! [cringe].... we'll pretend we didn't hear that. Hope you keep a spare firing pin around for when you bend/snap the one your using to scrape.

Heh  used the firing pin for years for cleaning for years.  It does not take any pressure to scrape the carbon.  You do not bear down on it at all.  

No need to cringe at all....

Quoted: Heh   I can do that too ! The plastic brush is a left over habit from the Marines...  I have always used a plastic brush on my M-16A2 and will continue to on my AR.  I also use the firing pin to scrap the little bits of carbon off when I am almost finished cleaning.

Ooooo.  Baaaaad.  Firing pin no like useie for clean!  Very bad.  No bone.

Even those comic book manuals said so!

Speaking of those...

I always wondered after the wisdom of having a woman say "Here comes shorty!" to troops in the field with reference to their weapon.  Hmmmm.

www.mrfa.org/images/m16manual/M16pages26and27.jpg

Of course, I guess it's no more concerning than the same blonde saying "Lube these parts generously and frequently..." several pages later.

I find a plastic brush, while too much work on it's own, works well enough if you soak the area (or the whole bolt!) in a dish of CLP for 20-45 minutes first.  (You guys!  You never soak dishes either!)