AR-15 - Groups Move/Increase with heat

[Ninsho]

I have an AR-15 that has been shooting great for me... a couple weeks ago I was shooting cloverleafs with it when I was doing my part.  Last week and today I've noticed the groups become shotgun patterns as soon as the rifle gets even a little hot.  Ragged holes to 10-20+ moa groups after 20 rounds.  I know this shouldn't be happening but I'm curious if anyone has any suggestions on what the cause could be?  I've tried two optics (a weaver fixed 4x scope and an aimpoint) as well as the iron sights, all do the same thing.  I retorqued the barrel today and that didn't change the symptoms either.  I'm shooting quality ammo (Prvi M193 55 grain rounds) that has gotten me good groups in the past.  I haven't tried different ammo so I suppose that could be the culprit.

Any suggestions as to the cause or how to diagnose it?

[techres]

Info please:

Pencil barrel?
Bull Barrel?
Gov't profile barrel?

Barrel Length?

Gas system: piston or normal?
Gas Block: normal front sight or modified?  Pinned or screws?

Curious.

[Aromatic]

Have you checked the barrel nut?  I had one work it's way a little loose once.

[Ninsho]

Its an M4 profile barrel (14.7" with pinned A2 flash hider).  Normal DI gas system with a standard F marked FSB.

Have you checked the barrel nut?  I had one work it's way a little loose once.

Yeah, thats what I meant be torquing.  I removed the barrel and reinstalled.  That sucker isn't loose now, and probably has a little over 90ft-lbs of torque on it.  Its a little high,  but I haven't heard of that causing accuracy issues to this extent... and Walk Kuleck's book states that he's gone over 90 plenty of times.

[Spartan]

Has the recent ammo been the same lot number of Privi ammo that was shooting well for you before? 
1. Privi is not really match ammo and you may have obtained a different lot of ammo that may not be as consistent as the lot that was shooting well for you. Bullets or powder may be "off" a bit....
2. Has the temperature of the conditions changed by 20F or more since the last time it was shooting good groups?  The ammo might not shoot as well in the heat as it does in the colder temperatures.
3. Have you cleaned the barrel very well to get the copper, and crud out?
4. How many rounds through the barrel do you have and is it a chrome lined barrel?  If not chrome lined you can have barrel wear after maybe 3,500 rounds that will effect accuracy-most notable at 600 yards.
5.  I assume that the barrel is 1:9 twist or faster if you are using 55 grain ammo in it.
I would try cleaning the barrel very well and I would try a different ammo...if you have a 1:7 or 1:8 I would try Hornady 75 match or maybe some blackhills 69 SMK or 77 SMK match and see if the groups come back.

[Ninsho]

Chrome lined barrel.

The ammo all came out of the same case.  The good groups were on the same day.  I'm able to make a ragged hole or at worst a 2-4moa group for the first group out of the barrel but 20 or 40 rounds later I'm shooting shotgun patterns.

The barrel has maybe 1000 rounds through it, it might be close to 7-800 rounds since the last time I cleaned it... but I can't believe that a little crud is going to turn a 4 moa group into a 20 moa group.

[SSG Platz]

Your barrel will change poa/poi as it heats up ( minimally, unless you are putting thousands of rounds down range in a short period of time) but your groups should not become iratic, whith the information provided it is possible that;

Something has came loose, possible your optics/ iron sights or even something else
Or could have possible been a bad batch of ammo, para has a decent reputation, but I have on occasion heard bad stories about it.

I hope everything works out, and when you figure it out let us know.

~PlatzOut~

[Ninsho]

My research so far has me inclined to believe that its possible the flash hider was over torqued when installed at the factory.  Unfortunately I can't loosen or remove due to the pinned and welded thing.

Again, three different sights all had the same result so I don't think my sights are loose.  Ammo doesn't seem likely to be the cause because of the  heating/cooling factor.  Barrel heats up, groups become not groups.  Barrel cools down and it shoots like a dream.  I wish I had brought some different ammo with me today so I could confirm it wasn't ammo related.

[SSG Platz]

I wouldn't think it to be the flash suppressor, could be the barrel nut though, also If you have your front sight on the picitinie rail you may be loose there.
I would check all the little stuff and if That doesn't work take it to a gunsmith and see what he thinks.

~PlatzOut~

[AFTERMATH]

Could an over torqued barrel nut stretch the receiver threads to such a point that it would cause erratic behavior only when heated?

[slim]

Is it possible that you're feeling the heat and pulling your shots?

Perhaps when it's cool you're taking your time and "doing your part" but then once that hand gets hot (or you're shooting "more than 20 rounds" - I'm assuming you're shooting faster/more as you go) or you speed up you're abandoning your good shooting techniques.

Maybe try to concentrate a little more on making good shots, even with a hot barrel and/or while shooting rapidly.

If you're sure you're doing your part then it probably is something with the ammo or maybe your barrel is toast. I'd have someone else - that I know is a good shooter - go a few rounds with it and see if the results are duplicated.

[SSG Platz]

That's actually a really good idea, I wish I would have come up with that and posted it first.

~PlatOut~

Is it possible that you're feeling the heat and pulling your shots?

Perhaps when it's cool you're taking your time and "doing your part" but then once that hand gets hot (or you're shooting "more than 20 rounds" - I'm assuming you're shooting faster/more as you go) or you speed up you're abandoning your good shooting techniques.

Maybe try to concentrate a little more on making good shots, even with a hot barrel and/or while shooting rapidly.

If you're sure you're doing your part then it probably is something with the ammo or maybe your barrel is toast. I'd have someone else - that I know is a good shooter - go a few rounds with it and see if the results are duplicated.

[Charles McKinley]

Platz you and Slim would be a dangerous combination! 

[Burnett]

From what I have learned, your barrel nut is WAY overtorqued.
30 to 80 foot/pounds and everything I've read has said that the closer to 30 you can stay the better.
Tighten to 30 and go the the next notch and that's it.

[SSG Platz]

From what I have learned, your barrel nut is WAY overtorqued.
30 to 80 foot/pounds and everything I've read has said that the closer to 30 you can stay the better.
Tighten to 30 and go the the next notch and that's it.

Normaly, and correct me if I'm wrong, but if you tighten your barrel nut to tight you loose overall accuracy, not good groups right off into terrible groups after a magazine or so. Which leads me to believe something is possible comming loose as he shoots rather than something is to tight.

~PlatzOut~

[Ninsho]

Right.  I would think overtorqing would give me consistent subpar groups not inconsistent from great to garbage and back again.

I contacted the barrel manufacturer and they've agreed its probably the barrel based on my description and I'll be shipping it to them to take a look.

[SSG Platz]

I would still suggest talking to a gunsmith, if you know a really good one around you, or one you trust or are pretty failure with, I built my own AR and have been around the platform forever so I can take it down and build it back but have not ran into this problem in the past but a gunsmith especially one that is formally trained and deals with AR's all the time may be able to tell you without even having to look at it. Could be as simple as you have a junk barrel and need a new one. It happens you know, my ar with all the fancy crap I have on it is only 4 moa, if I'm lucky, with M855 rounds a little better with MK262, but it's consistant and for close up that's cool, my next project is to build a Mk26 upper built around a really good barrel.

~PlatzOut~

[Yuppiejr]

The part I'm having a hard time with is the return to accuracy when the rifle cools... if it were a mechanical / loose component you'd think it would remain inaccurate even after it cools.  Was this a brand new upper/barrel or something you picked up used?  Care to share the manufacturer?  How are you cleaning, sectioned or solid rod, bore snake, chemicals, frequency, etc...

I've never torqued a barrel to 90 ft-lbs, as others have said, 30 pounds and then index to the next hole for the gas tube.... however I can't see how this would impact group sizes like this, I've only seen this crack receivers or cause canting of the front sight.  Since you've tested with optics I'd rule out the front sight.

Basically I'm down to you having a lot of Prvi M193 with undersized projectiles or a bum barrel/flash hider...  If you have calipers you could measure the diameter of the projectile at it's widest accessible point (just ahead of the case neck) and see if it measures .224".  You can also "slug" your barrel using some egg shaped lead fishing lures just slightly larger than .224 inches.  You want to take 3 readings:

Run the lure into the first inch past past the throat and then pound it back out from the muzzle with a sturdy cleaning rod.  Measure the diameter of the weight.
Run the lure into the first inch from the MUZZLE end, and then pound it back out from the breech with a sturdy cleaning rod.  Measure the diameter of the weight.
Run the lure from the breech to the muzzle and let it pop out the end, measure the diameter.

Ideally, all 3 will measure the same.  If you find a variance I'd suspect you've found the problem... it would be interesting to take the 3 measurements on a cold and "hot" bore after firing a few shots.  If the barrel & ammo measure correctly I'd suspect the flash hider which is obviously a problem unless you can un-pin and replace it...

You could buy one of the clearance barrels from Palmetto (FN makes em) + a midlength gas tube & and A2 flash hider:

http://palmettostatearmory.com/index.php/ar-15-05/barrel-clearance/16-5-56-nato-1-7-chrome-lined-chrome-moly-vanadium-hpt-mp-mid-length-gas-heavy-profile-barrel.html

... depending on gunsmithing cost to get the barrel headspace adjusted/checked via the extension it might be just as easy to buy a new upper.

http://palmettostatearmory.com/index.php/ar-15-05/complete-uppers/m4-t9-madness-16-cmv-chrome-lined-stripped-railed-upper-without-bcg-or-charging-handle.html

Sorry to hear about the trouble!

[TN_Dadx5]

How is your sling attached at the muzzle end?  If it is on the gas block, the combination of too much torque and sling pressure can jump the hot barrel from shot to shot.  Try re torquing (possibly with a new nut) and shooting w/o the sling. Other than that, it may be a cracked upper.

Charlie

[SSG Platz]

How is your sling attached at the muzzle end?  If it is on the gas block, the combination of too much torque and sling pressure can jump the hot barrel from shot to shot.  Try re torquing (possibly with a new nut) and shooting w/o the sling. Other than that, it may be a cracked upper.

Charlie

It would have to be a really light barrel and a whole lot of toqre, to move that thing in the fashion he is talking about, a sling or sclilencer will "droop" your barrel causing your poi to change throughout the day as you shoot, but it should stay consistant during each SOF.

~PlatzOut~

[Yuppiejr]

Throwing the rifle in a lead-sled/mechanical rest or at least shooting from a bench with sandbag support (we're testing equipment here, not marksmanship) to remove as much of the "user variables" from the picture as possible should eliminate sling torque or other factors.

I would also consider the gunsmith option, while I'm pretty comfortable messing with the platform I've also never seen a behavior like this...  try some of the stuff above  (check the bullet diameter in your ammo, slug the barrel, shoot from a rest/sled, etc..) but if nothing obvious pops up it's likely beyond user repair. 

If the receiver is trashed it's a relatively inexpensive fix, if it's the barrel I expect parts & gunsmith time will be $250+ which is while you can pick up a new complete upper (you would move your BCG/charging handle over) .. example:

http://www.dsarms.com/Lowers%20&%20Uppers/products/59/

[SSG Platz]

Couldn't have said it better, infact ransom rest was going to be my next suggestion for self diagnosis.

~PlatzOut~

[Ninsho]

The manufacturer (CMMG) is shipping me a new barrel.  They still don't know the cause but confirmed that the barrel wasn't producing satisfactory groups on their end.  Hopefully they'll let me know what it was when they figure it out.

Meanwhile my barrel troubles continue with another barrel from a different manufacturer appearing to be out of spec on arrival yesterday.  (Bad headpspace and rail won't fit on the barrel nut) 

I think I'm going to order everything in triplicate from now on since no one gets it right on the first try.  So much for 'top shelf' AR15 manufacturers.   

[jrturbo]

As it is a chrome lined barrel I would believe that it is likely a button rifled barrel.  Sometimes the post buttoning stress relief is insufficient to keep a buttoned barrel from moving when it heats up.  Or possibly the odd chance that there is a flaw in the steel that is unseen.  I have seen barrels that will shoot very good groups in slow fire and then fall apart during rapids.  I commend CMMG for standing behind their products.  When it is a flaw that would cost hundreds of dollars to discover using destructive testing to find (or a costly x-ray session that may or may not detect stress) sometimes it is better to just replace the barrel. 

If you want to shoot small groups consistently get a cut rifled barrel.  A quality barrel will last you about 5000 + rounds.  (With superlative accuracy until  about 2500 rounds. By superlative I mean .5 MOA or better.) 

Turbo

[Spartan]

Compas Lake Engineering upper with a Krieger 1:7 twist

[hinermad]

I think I'm going to order everything in triplicate from now on since no one gets it right on the first try.  So much for 'top shelf' AR15 manufacturers.   

There's been a lot of demand for AR pattern rifles and .223 / 5.56 ammo lately. I wonder if the manufacturers are letting quality slip (unintentionally or otherwise) to meet the demand.

Dave

[Ninsho]

I'm starting to wonder if the barrel is actually fine (the new one) or if its the rail I'm trying to put on it.  Anyone have a troy mrf-di mid length they can measure a few dimensions on?

[Ninsho]

Or a few dimensions on a mid length barrel for that matter...

[franklinfarmer]

Just to let you know, I seem to have exactly the same symptoms on a Colt H-Bar (22" barrel).  First 5 rounds (2MOA group from prone).  Second group 3 or 4 MOA.  10 rounds (stage 1) 5 MOA.  Next 5 rounds (all called good or close) 12 MOA "group."  Next 5 or 10 rounds 18 or 20 MOA group!  Let it cool down.  Back to a 4 MOA group.

Edit/add:  Groups also move up and right as much as 12 MOA with warmth.  Some movement up and right is typical of the nut behind the stock in this case, but not that much.

This is iron sights.  Everything seems solid.  Best guess so far is heat causes something with the barrel mounting to move.  But I'm just guessing.

I have solved the problem by replacing the barrel with an M1A barrel.

[SSG Platz]

Try moving closer to the targets, and work no your fundamentals.