AR-15 Question -- Wierd & Frustrating Problem

[Daniel Nighteyes]

The short form -- my (very new) AR-15 will reliably consume and cycle any ammo I feed it, with the exception of Sellier & Bellot match ammo.  With S & B, pulling the charging handle will chamber a round every time.  However, when I fire the rifle, the next round in the magazine almost never chambers.

I have to pull the charging handle to chamber the next round.  Bang -- pull -- bang -- pull, etc.

Again, this is ONLY with S&B match ammo, of which (naturally) I have nearly 1,000 rounds.  I've measured the rounds, and they're within spec, albeit slightly on the small side.

Any ideas as to what's causing it?

[Dirty Brass]

I'm no expert, but I'd say the S&B is underpowered if everything else cycles the bolt and picks up a new round.

[August]

Have you tried using the S&B with different magazines?

Exactly where does the round that doesn't chamber end up?  Is it still in the magazine?  Does it end up wedged against the feed ramp?  Does it end up wedged in the chamber?  Does it end up part, but not all the way in the chamber?

Are you running lots, and lots, and lots (you get the picture) of oil on the BCG???

What are the tips of these boolits like???

I'm gonna guess that all your other ammo is right on the edge of not working also.  The S&B is probably giving you information about the overall situation of lubrication, gas system integrity, buffer spring condition, etc.

It's most likely -- in this order -- the magazine, insufficient lubrication, loose gas key on carrier, misalignment of gas tube to gas key, out of spec buffer spring, pinched gas tube at front sight and/or barrel nut.

You don't mention the brand of AR.  That might also be helpful diagnostically, since some brands have patterns of failure.  It is also important to know the type of rifle: is it a 20", full length gas system, a carbine with short gas system, or a middy with the in-between length (mostly rock rivers).

Answer these questions and we'll go from there.

It's most likely a simple thing, so hang in there.

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[Daniel Nighteyes]

It's a Stag Arms left-handed A2-style upper mated to a Stag Arms A2 lower,  20-inch heavy barrel with a California-compliant muzzle brake. It's possible that I don't have quite enough lube in all the right spots, and it is nearly-new.  I have tried several magazines (10-, 20- and 30-round) with exactly the same result, so we can pretty well eliminate magazine problems (I think).

The ammo that does cycle the action is some PMC ball ammo that I've had for at least 20 years, plus Black Hills Heavy Match, and Winchester Heavy Match.  The failure-to-feed round is usually left in the magazine, but every now and then (once in every 50 rounds or so) it gets crossed up in the action.  This typically happens with the last couple of rounds in the magazine.

[August]

If it's left in the magazine, then the bolt is not cycling back far enough to pick up the next round.

First, check the gas key, which (as you may already know) is the gooseneck looking thing that's on top of the bolt carrier.  Be sure is is tight and also that it is lining up perfectly with the gas tube when the action is closed.  This will insure that whatever gas is coming back from the barrel is getting to the bolt.

Next, have a friend help you remove the hand guards -- one person cannot do it alone.  Check the gas tube for alignment as it goes across/though the barrel nut where the barrel meets the receiver.  It should be in a straight line and not bent at this point.  Then, check it at the front end where it goes into the sight base to be sure it isn't pinched.

Stag make a high quality product and a twenty inch barrel has plenty of "dwell time" with just about any load.  In other words it should work fine with the S&B loads.

Check the bottom of the bolt carrier to be sure it has no burrs.  It seems the friction of the cartridges in the magazine with the bottom of the bolt is an issue since the gun gets closer to running when the magazine gets toward empty.

Then, clean and oil it with lots of oil on the carrier and the piston part of the bolt.  While you've got the bolt and carrier apart, check to be sure that the three piston rings on the bolt are arranged so that the break in the rings does not line up -- stagger them so that the openings are each 1/3 of the way apart from the others.

If it doesn't run after all this, then I think you contact Stag and see what their reply is.

I hope something here helps!

Good luck.

[Daniel Nighteyes]

Thanks, August.  What you say makes excellent sense, so I'll get to work on it next week. ( BTW, I can single-handedly remove the handguards, 'cause I've got the removal tool.  Works great!)

-- Nighteyes

[saddler]

In addition to what "August" advised, it may also be a case of the gun NOT YET being run in/broken in yet

I'd also consider running 500-1000 or so of the cheaper bulk pack rounds through it, then see if the S&B works after the gun has had a good clean/strip down

S&B ammo has always had a great reputation here in Europe for not only being good & reliable, but also for being on the "hot" side (quite a bit of poke to it)

IF its as new as you think, I'd also carry out the full barrel break in routine to avoid barrel related issues & make future cleaning chores easier

Jon

[Daniel Nighteyes]

Well, after several months and lots of effort, I have come to the conclusion that the problem is with this particular lot of Sellier & Bellot ammunition.

Nothing was wrong with the gas tube, etc.  As my rifle (and the barrel, Jon) got progressively more broken in, the Sellier & Bellot ammo did begin to cycle better -- to the point that the "bang-click" phenomenon was greatly diminished.  However, I'm still having about one or two rounds in every 20-round box that won't cycle the action sufficiently for the next round to chamber. Instead the next round gets crossed up and stops the bolt from closing.  The marks on the brass suggest that the bolt hasn't cycled properly.

As a test, I've also run Remington, PMC, Winchester heavy match, and Black Hills heavy match ammo through the rifle.  All of them  have consistently fed, fired, cycled the action, and ejected properly.

Regards,

-- Nighteyes

[Dirty Brass]

Yup - like I suspected. Underpowered ammo. It happens even from the factories. Glad you set your mind at ease and things are working better. I've shot a lot of AR's, and most of the time cycling problems are magazines or ammo - the latter being uncommon unless reloads are being used.

[Daniel Nighteyes]

... and most of the time cycling problems are magazines or ammo - the latter being uncommon unless reloads are being used.

Forgot to mention that I used three magazines in the testing -- the plastic 10-rounder that came with the lower, a milsurp USGI 20-rounder (I replaced the spring and follower), and a somewhat newer milsurp USGI 30-rounder.  The mags functioned flawlessly with all but the S & B ammunition.

[Dirty Brass]

Sounds like you were thorough. All's well that ends well! 

[WyrTwister]

     Only other thing , if you are good enough gunsmith , is to remove the gas block / front sight base .

     The gas port in the  gas block / front sight base is usually larger than the gas port in the barrel , to make up for slight misalignment .

     There may / probably will be a gas " stain " on the barrel , corresponding to the size of the gas port in the  gas block / front sight base .  See if this " stain " is pretty well centered around the barrel gas port ?

     Now , have you been saving your brass , so you can reload it ?

 

God bless
Wyr