Odd vertical grouping

[usmarine0352_2005]

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First a little background. My department ordered quite a few S&W MP15X rifles. When we got them some of them just would not sight in correctly. We checked the ammo and rifles and determined that some of the rifles chambers were too tight.

After much back and forth we found out that S&W had purchased Thompson/Center and were in the process of moving Thompson/Center's machinery over to the S&W factory (or so we were told). In order to boost production and fill orders S&W farmed out the barrels and purchased a bunch from Mossberg. The batch of rifles we had came with some Mossberg barrels and some Thompson/Center barrels. I believe the Thompson/Center barrels have a "-X" on the barrel while the Mossberg barrels have a "xX" or something similar. So the barrels that were having problems with are the Mossberg barrels.


So S&W sent us all new Thompson/Center uppers. We went to the range today and changed out the uppers and all of them shot fine, except mine. Mine made ok groups at 100 yards but they seemed to string up and down vertically from about 5" to 10" apart. I'm an ok shot but then two of the range masters shot (both of whom are excellent shots) and they had the same problem. Nice groups but strung vertically up and down, no flyers to the right or left. So the windage was fine but the groups were moving up and down vertically. (I should note that it seemed that a few of the new upper's front sight tip was not in a locked position out of the box but rather right in the middle. That was fixed before shooting.)


I put on another new Thompson/Center barrel and the problem was solved.




Anyone ever heard of this problem?




I put this in General because perhaps this is a problem that people have had with pistols or shotguns moreso then rifles.

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[Trent]

Inconsistent velocity is the most prime suspect in vertically stringing. Bullets leave at different velocities, different times of flight, gravity has longer to affect some than others.

If the barrels are crowned incorrectly, that could cause it, but that normally leads to widened groups, not vertical stringing.

Gas port or incorrect throat seems more likely. If bullets have to jump a gap to engage rifling that'll cause inconsistent velocity. If the port / gas impules has any flaws that could cause it also. I had an AR barrel that shot very inconsistent, found a burr during manufacture folded back in and obstructed the gas port. (Found this after I grew suspicious and pressed off the front sight post to check if it was aligned properly). Got that chunk o' chrome out of there, and it worked great afterwards.

But, could be any number of small defects or combination thereof.

 

[Boo-Radley]

Sometimes "stringing" is caused by inconsistencies with the ammo.

 

[usmarine0352_2005]

Sometimes "stringing" is caused by inconsistencies with the ammo.

You'd think.....but the same ammo worked fine in the next upper and all of the other guns.

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[Trent]

Definitely, could be, if the powder wasn't dumped consistently or has other defects the velocity would be different (same effect, different cause).

I'd didn't think of it since the problem went away with a barrel swap - I didn't assume they'd switch brands of ammo in between. But they might have.

Edit; nevermind, they didn't switch ammo.

 

[Trent]

The good thing is, if they swapped parts and the new ones work, they'll keep working. Whatever the manufacturing defect was (as I said before I suspect it was a gas regulation obstruction or throat machining), it'll be gone for good.

Just be careful of the crown, and they should stay accurate.

 

[usmarine0352_2005]

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Any other ideas?
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[9mmepiphany]

perhaps this is a problem that people have had with pistols or shotguns moreso then rifles.

When this happens with pistols, it is almost always a result of varying grip pressure...I don't think this carries over to long guns

 

[jerkface11]

What position where you shooting from? What kind of sight were you using?

Inconsistent velocity is the most prime suspect in vertically stringing.

Inconsistent velocity would open the group up but you'd have to be using vastly different loads to get anything I would call stringing.

 

[KodiakBeer]

Stringing in a bolt action rifle would be due to improper bedding and is most commonly seen in thin barrels. The groups don't go up and down, they generally just string higher as the barrel warms.

In an AR? I don't know, but I wouldn't blame the ammo.

 

[Trent]

Man, I have to retract my statement here.

I just ran the numbers and in order to get vertical stringing THAT widespread (5-10" according to OP) you'd have to have MAJOR velocity problems. To the point you'd notice a significant difference when you pull the trigger.

Assuming a 50/250 yard zero, at 100 yards, a 55 grain FMJBT with a BC of .245, you would need *500* feet per second (3000 vs 2500 fps) difference in shots to move the point of impact 2.8 inches. That's HALF of what the OP said. To get a > 5" difference in point of impact, you'd have to launch two projectiles out - one at *2000*, and one at *3000*.

That is NOT a gas system problem.

That's a LOOSE PART problem.

Question is which one?

And will it happen again?

 

[usmarine0352_2005]

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Someone said it could be a loose barrel nut, could that be it?



I don't even know where the barrel nut is.

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[Trent]

A loose barrel nut would almost certainly open groups in all directions, not just one axis.

I'm not familiar with the internal design of an SW MP15 or what changes they made from the stock AR design. On stock AR's the barrel nut is attached to the front of the upper receiver. It is under (inside) the D-Ring on for grips, or the freefloat tube if freefloated. So it's located "inside" the D-Ring or inside the very back end of a freefloat tube.

With it being uni-directional group disbursement I would think the likely culprit (after my first retraction), may be a lose sight issue.

But if the barrel was loose, it would cause the front sight to move, sure, but if it's loose the chances of it staying in a straight line vertically are pretty slim - it could move in any direction.

You said the front sight was loose in one post. Is it possible to remove and check the threading of the front sight post? A bad tap job and that front sight might not stay in one spot under recoil.

Just a disclaimer, but lots of variables at work here, and without getting hands on to examine and measure things, everything you get over the internet is going to be conjecture at best.

 

[Redlg155]

The best explanation that I could offer is that there may have been excessive vertical play in the fit of the upper and lower reciever. Firing the weapon may have resulted in inconsistent reset or flex.

 

[usmarine0352_2005]

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They noticed a few of the new uppers had front sight tips that weren't in a locked position. Actually the 2nd new Upper I got was like that. It was put into a locked position and fired fine.



Very interesting.

 

[bigedp51]

I know the British Enfield rifle is not a M16/A4 but here goes.

Enfield rifle with two piece stock.........

Vertical stringing is caused by a loose fore stock.

Horizontal stringing is caused by a loose butt stock.

Your problem was cause by a loose fitting part caused by a company bean counter who decided to use sub standard parts and not not listen to the people in the field. Its called greed overriding quality control..............

When Remington bought Marlin and quality control went to the dogs the bean counters had a big part to do with this.

97% of all errors are human errors and only 3% are actual mechanical failures, someone at S&W dropped the ball and quality control inspection was lacking.

 

[usmarine0352_2005]

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So your saying the AR15 forestock was most likely loose?

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[bigedp51]

usmarine0352_2005

What I'm saying is simple, the instant you pull the trigger chamber pressure and gravity cause the barrel to start flexing downward, when the barrel reaches its elastic limits it starts flexing upward. Accuracy is "controlling" barrel flexing and vibrations.







Top photo, loose fore stock on No.4 Enfield rifle, bottom photo, tight fore stock properly bedded with 7 to 9 pounds of up pressure at the fore end tip used to control barrel vibrations. (10 shots, 50 yards)



Vertical stringing is caused by something being loose and the barrel flexing uncontrollably. You replace the entire upper to cure the problem, BUT I will take a WAG and say barrel nut torque or defective receiver that allowed barrel flexing.

 

[jerkface11]

Could be an over torqued barrel nut too.

 

[usmarine0352_2005]

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Ok, I'll pass it on.
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[jerkface11]

It uses a free float tube correct?

 

[usmarine0352_2005]

Not sure. It's a SW MP15X.