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Discussion Starter #1
Hello all. I recently came into the possession of a 20" AR15/M16 barrel I'm going to be using for my M16A2 clone upper. I am pretty sure it is FN manufactured but want to make sense of the other markings and be sure it isn't a reject barrel or something else (knockoff).

The seller tells me he inspected the barrel and expects there to be no more than 200 or so rounds through it. The seller was previously an armorer at a Marine Corps base camp and was told to get rid of extra parts. Many he said he kept and this was one of them. He says this was in the overstock pile of parts and for some reason or another they couldn't hang on to too many extra parts. It's possible they just got new barrels and switched barrels like these for those, he says. He was very upfront with me so I am inclined to trust him, but just want to make sure.

I shone a light through the bore and it does look pretty clean. There is some light surface rust (nothing that is not fixable with some CLP and 0000 steel wool) and some sections rubbed silver.

Here are the markings and how I interpret them:
The first marking reads "FNMI MP 5.56 NATO 1/7" - FN manufacture, ?, 5.56 Nato caliber, 1 in 7 twist
20210407_194009.jpg

Then, "9/04" - Manufactured September of 2004
20210407_194026.jpg

Then, simply "8" - ?
20210407_194036.jpg

I guess my concern would be a proof-marking? I know any barrel is shot with at least one bullet that is overcharged upon manufacture. Then it is visually and magnetically inspected for cracks and the like. I think I read somewhere that there should be a "P" stamping somewhere to indicate that it was proofed, but maybe my memory is failing me. Also, if only 200 rounds were through it, why would it be rubbed so much in certain spots?
 

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MP = Magnetic Particle (magnaflux) tested. My question would be if it only had 200 rounds through it, why was it removed from the original rifle? How does it gauge?
 

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Discussion Starter #3
I don't have a bore gauge - seller says he checked it out and it was in very good shape. He estimated around 200 rounds through it. Visual inspection shows it seems that is correct.
 

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Just how do you estimate round count ? Unless you owned it & counted how mush you shot it , its just a guess & not a very good one . Probably an A2 barrel , Rustic look to it , in storage for a while ? Flood ? hard to say , maybe just not taken care of .
 

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Discussion Starter #5
Just how do you estimate round count ? Unless you owned it & counted how mush you shot it , its just a guess & not a very good one . Probably an A2 barrel , Rustic look to it , in storage for a while ? Flood ? hard to say , maybe just not taken care of .
That's what I would guess - in storage for a while after it was taken off. It is a barrel from 2004 after all. Probably just sitting around. I had a Savage 110 with worse rust and it cleaned up nicely. I did this barrel last night and it looks good. Interior does have some copper fouling, though (but so does my M14 and that shoots fine).

I guess my reasoning for making this post is to confirm simply that it is indeed a USGI FN barrel and that it was inspected by the manufacturer at the very least to not have cracks and the like. Also, I know some Colt barrels will be marked chrome moly to indicate a chrome-lined bore - would that be the case for FN? That is something I would also like to know.
 

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Every year around October when budget time rolls around it's a mad dash to devalue supplies & equipment or get rid of any overages, or, those items a unit isn't supposed to have on their books so they can try for a bigger slice of the budget pie. We also used to have something called TO&E and during inspections you were only allowed to have what was on the books, anything else was hidden or tossed, I saw some insane things get thrown into the pond behind base.

It does indeed look like a legit FN barrel to me, as to why they changed it is anyone's guess, it could have been for something like it failed a no go gauge, got bent, or, could have been for nothing because some jarhead couldn't shoot! You'll just have to try it and find out.
 

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Discussion Starter #7
Guess we'll have to see - good thing I have some G/NG gauges on the way. Barrel does not look bent to me.
 

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It could have been for anything, I wouldn't worry too much about the headspace as that's determined by the locking lugs on the breech end, it's either good or it ain't, can't be adjusted by any means I'm aware of. I'd be more concerned about the barrel or FSB being out of kilter, and possibly excessive adjustments to get it to zero, or, some jarhead could have used his rifle for a prybar, trust me it happens more than you know! If you can't physically see anything you'd need a straightness gauge to drop down the bore to find out, personally I'd just install it, zero out the sights, then bore sight on an object to see if it's majorly off.
 

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Discussion Starter #9
That's the plan - good thing I have a spare upper and I didn't pay too much for this.
 

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Discussion Starter #10
So I got a set of GO/NO-GO gages today and checked the barrel headspace. GO (1.4636) test works fine. NO-GO (1.4666) test did not. GO gage was able to turn freely while NO-GO gauge was quite a bit more difficult to turn but still turned. My process was to remove the bolt face but leave the extractor/ejector on. Then, with the barrel in a vice I insert a gage and apply forward pressure to overcome the ejector spring. From there I turn the bolt.

I have read in some spaces that failing a NO-GO test is not the end of the world and that it would be acceptable to use should it not fail a FIELD gage test. However, would precautions have to be taken should this barrel be installed before firing? At this point I am considering buying a new barrel. However, if the situation is simply that the barrel is acceptable to use but may not be as accurate as a new barrel, that would be OK with me as I could always use this barrel and then get a new one down the road.
 

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Discussion Starter #11 (Edited)
Little update to this - I am using Forster 223 Rem gauges by the way. I put the barrel into the upper receiver, and retried the tests this time pushing the back of the bolt carrier forward into the gauges within the chamber (no extractor). The GO test passes no problems. The NO-GO test does not pass (bolt closes), but I really have to apply a good amount of pressure to the back of that bolt for it to close. Taking the bolt out again is equally difficult.

Also, I am using a RRA bolt carrier group in this build. Should I swap it for something closer to the barrel?

I purchased a Forster FIELD gauge now and will test when that comes in.
 

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If I remember correctly the .223 " No Go " gage in a 5.56 Chamber may fit the way you describe , I use the Stripped Bolt to do all HS Checks . You shouldn't force an HS Gage . It's up to you , if you think too much HS , don't use it . I think it's OK to use , but thats IMO .
 

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Discussion Starter #13
I have a feeling this will be a fine pick-up-and-go barrel with acceptable, but not fantastic accuracy - which is totally fine with me. Once the gun shortage is over I will pick up a stainless barrel if I want.

I ordered 5.56 FIELD and MAX gauges which I seriously doubt will allow the bolt to close. However, I won't shoot this until I have the gauges in hand and can test. Thanks all for the advice. I will update once I get them in.
 

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Use your best judgement, but, if it were mine I’d install it and test fire, that barrel might end up being a keeper.
 
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