I bought this, old 16" "C MP B" stamped barrel, it was advertised as used, had some obvious external rust, freckling mostly from pics listed with it, but once I received it, I noticed there is a pretty decent spot on the face of the barrel inside the extension.
I have the spot coated in CLP right now, I had pretty decent luck last year with it on a couple full curve mags that had less than mint floor plates.
I figure I'll give it a couple days, then I have one of the chamber cleaning tools/carbon scrappers, I'll run a couple times.
I know the vinegar trick, any other suggestions appreciated.
For some reason, it had the FSB removed then reinstalled later, the front HG retainer is about 45 degrees off and DOES NOT MOVE.
I'll have to knock the pins out to adjust.
I figured it wasn't perfect, got it for a GAU-5 type project, so wasn't looking for something minty, and I don't think the seller knew it had the spot on the face, auction house that clears stuff out as quick as they can, so don't fault the seller.
Maybe I'm being too picky.......
Attached File

Attached File

Attached File

Attached File

Attached File

brass brush, oil, elbow grease.

Doesn't look bad. minor even.

Your bore and chamber is chrome lined so should be good to go.

bore cleaner, rub hard

Rust through parkerizing can’t be removed without removing or damaging the finish.  The rust you see is leeching from the surface metal up through the parkerizing.

Best bet is to send it off and have it bead blasted to remove the rust and have it reparkerized.

That's news.
I did not know that.
Of course, you wouldn't think there would be a "correct" number of times a FSB could have the pins punched out and replaced, because.....there......wouldn't.....be......a.....REASON to do it once installed, but "once and done" is interesting.

Never heard this before. Source?

I've always reused so long as the pins are visually GTG.

I would use a drop of oil and some 0000 steel wool in the rust areas very lightly or you will remove the finish. I have used vinegar to remove rust on old animal traps, oil lanterns, old tools, and many old items. It works great removing rust. On very delicate things, I have used Evapo Rust. Oil part right away or it will flash rust again. (I use boiled linseed oil on animal traps, oil lanterns, etc.) You can also re-park it yourself. https://www.palmettoenterprisesparkerizing.com/product-page/manganes-pint-16oz  Heat to 200 degrees and soak part in for 20-30 minutes. I also have a blueing sharpie for small places - https://www.amazon.com/dp/B000TTIEY6?tag=arfcom00-20

I have had a few where the HG cap was frozen like that. Let it soak overnight in oil then a few taps with a rubber mallet, it broke loose. It is just a few rust partials got it frozen in place. Even a soak in vinegar or Evapo Rust may break it loose.

I am also a logic kind of guy. I can get the BCG key screws are a once and done. Stretch the threads and it may not torque right the second go around. I get a crush washer may be a once and done. Once its flattened, it no longer locks the same. I get the Delta / Slip ring snap ring has a sharp side and goes on one way. I get certain parts develop a wear pattern and needs to be installed back same direction. The front taper pins, one use and done, I have never heard of before. I can even understand roll pins and lock washers may lose a little spring once used. But a solid metal tapered steel pin is a solid metal tapered steel pin. I do not understand how they would not have an infinite life.

Side note: A lot of us in the retro building, like me, have a small spray bottle of vinegar, table salt, and hydrogen peroxide that we use to add a little yellow, brown, rust tinge to some of our parts to give it the look you have now. I would not want any rust on my chamber area or any working area, or even making deep pits in my parts, but a good wipe down in oil, that barrel looks pretty cool to me.

Soaking in CLP for a couple days, then hit it with a stiff bristle brush, worked pretty well.

Before:
Attached File


After:
Attached File


I did pop the pins out to adjust the front HC cap, then got it pieced together.
I had a very used and abused Colt slip ring unit that had been painted black, might have been a PR gun, or some knucklehead in the "Survival Days", hit it with hot soapy water and the aforementioned brush, got 95% of it off, didn't want to remove the anodizing, so called it good.
The HGs are really tight, then again, it's pretty difficult to manhandle that thing, it's easy to see why they came up with the delta ring.
Slamming the bottom HG at the rear helped square it up better.
Attached File

Attached File

Attached File

Attached File

Attached File


Next, putting these all together.
Attached File

Attached File

AR15sport pretty much took all the "gun budget" for a bit. Not cheap, but WAY more affordable than the real stuff.
Besides, most of my builds just "identify" as retro, so....