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I've had a chance to fire my PolyTech M14S that I snagged at Knob Creek. The rifle is in excellent condition, and appeared to have been unfired by previous owners. Bore is chrome-lined and the muzzle measures 1.0. I'm getting a left-to-right, and upward diagonal stringing of successive shots fired from a cooled barrel. See the targets below. In each case, I let the barrel cool with the bolt open for at least 20 minutes between firing sessions, then fired five rounds fairly quickly over a period of about 20 seconds (4 seconds between each shot).










In this last target, I mounted a full 20-round mag, and with about 4 seconds between each shot, I fired the first fifteen at other targets, and the last five at this one. It seems that as the barrel gets hot, the stringing "calms down" somewhat, and the rounds fall in the same general area.





It seems that I have heard of M14's and M1A'S suffering from diagonal stringing in the past, but I don't recall what any of the remedies were. Do any of you guys have any ideas as to how to correct this condition?

-Bean
 

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Most likely a stock/bedding problem, with other M14 specific possibilities.

Try registering here and doing a search: http://www.m14tfl.com/upload/index.php

From a Warrfiles.com thread on M1A vertical stringing:

Well, you'll need to rule out some easy stuff first:
shoot with match ammo
check the gas plug is tight
make sure the rear sight tension is good (although IME, the aperature will drop, not raise when there's not enough tension).
make sure there's no play in the flash suppressor and check the inside for copper streaks (this usually results in a big messy target, but check it anyways).
If you're shooting with a sling, if the sling slips or the tension gets less, I wouldn't be surprised to see some minor stringing.

Now it's a matter of carefully checking the rifle over in a methodical manner so that you don't trample the evidence in a rush to pull the rifle completely apart. Think about any parts you may have swapped out.
The handguard should not contact the stock. There should also be a slight gap between the handguard and the receiver so that it won't get pinched when the barrel warms up. I know you ruled it out, but check it anyways.
Take a look at the gas system. Make sure the front face of the stock ferrule is not contacting the face of the front band (the ferrule should pull downward on the band, not push it forward).
The trigger group lockup should be tight--last 1/4" when closing the trigger guard should take some pressure.
Check the inside of the stock and make sure the oprod is not rubbing wood.
Make sure the unitized gas system hasn't loosened up (ie screws loose).
Check the bedding surfaces for chips or other degradation.
 

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Do you, by any chance, have a bipod attached to that rifle? It would appear that something is affecting the barrel. Do you have the original Poly stock on it?

Barrel supported bipods are worthless so don't use one. IF you have one that clips onto the front ferral, it's possible that the barrel is touching the threaded bolt-end of the ferral that sticks up into the inletted area and needs to be ground flush. There is not much clearance between the barrel and the lumber.

If none of that is the problem, I'd too agreed that it's the bedding. Something is bothering that barrel. I've built a match-grade shooter using a Poly and it was dead-nuts. Make sure that the stock isn't touching the barrel anywhere by slipping a piece of paper under it between it and the stock from the muzzle as far back as you can. The barrel should be floating in that inletted area. I'd also suggest you pick up a Fred's M14 stock. Buy an "excellent' one and it'll cradle that hardware perfectly and I'll bet it'll correct your stringing. That poly stock is poor compared to the M14 stock.

Rome
 

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Discussion Starter · #4 ·
Thanks guys,

It sounds like I need to look at the stock fit. I may even go ahead and order a USGI one from Fred's.

-Bean
 

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Muzzle crown uneven?...I've had rifles I swore had near excellent muzzle crowns, but with problems similar to yours, inconsistent shot placements, excessive changing of point of impact with heating, inaccurate with almost all loads except one maybe two loadings, but then I recrowned the muzzle myself with simple methods of a round headed screws chucked in a hand drill, smeared with valve grinding compound to make the round headed screw head make a nice recrown, but hey, who knows for sure, just I do know I wasted alot of ammo and years on several .308's that I refused to believe the muzzle crowns were cut improperly, and when you are dealing with century arms products, or other low end items, it happens, especially to bargain basement pistols of the past, like my helwan 9mm and tok 9mm chicom that both had brand new bores and muzzles, shotgunning shots all over the paper at ten yards till I recrowned to make shots going into previous holes in the paper.
 
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