NON-Square Casehead Effects on M1A/M14 Accuracy?

[Grump]

Has anyone tested or have any knowledge of what effects, and how much of an effect if there is one, out of square caseheads can have on accuracy in the M14-pattern rifle?

I ask because a few months ago, I stumbled across a pair of old articles in the American Rifleman by Creighton Audette concerning the, um, "ability" you might say of two-lug bolt guns to throw bullets into two groups 1 MOA or more apart, if their caseheads are not square and their high spots are aligned in the chamber at 3:00 or 9:00 (as in horizontal) orientation.

The articles were Cartridge Cases and Accuracy, Creighton Audette, American Rifleman, June 1981, p. 27 and January 1982, p. 30.

I'm now looking for:
Overlooked Aspects of the Cartridge Case, Creighton Audette, Precision Shooting, Dec. 1981. p. 21; Highpower Target Rifle Shooting, Creighton Audette, Precision Shooting, September 1986, p. 4).

There's kind of a summary of all that here, halfway down the page: http://riflemansjournal.blogspot.com/2009_07_01_archive.html

Anyway, for those bolt guns, up to .002 casehead runout was no obstacle to good accuracy with nice round groups. More than that and things got interesting.

I tried to test it a month or two ago, but did not have many cases with .003 or more out of square, and the accuracy results were inconclusive.

Supposedly, three-lug and multiple radial lug systems (like the AR Series) are much less susceptible to having accuracy effects.

Thanks!

 

[Grump]

Re: NON-Square Casehead Effects on M1A/M14 Accuracy?

BTTT.

 

[johngfoster]

Re: NON-Square Casehead Effects on M1A/M14 Accuracy?

I have wondered about this as well. Could this be why Savage rifles tend to have a slight edge over Remington for out of the box accuracy? They have a "floating" bolt head and I was wondering if this could be why.

 

[Grump]

Re: NON-Square Casehead Effects on M1A/M14 Accuracy?

Well, I understand the operating theory and Mr. Audette rather thoroughly documented the threshold at which the effects would show up in two-lug bolts with the 0.473 casehead.

I'm not aware of him or anyone else testing it in any other system. He mentioned an *expectation* that three-lug and radial-lug systems would either show no dispersion from THAT variable, or would show it only if lug bearing were uneven.

I suspect that my son's long-sold-off Grendel upper that would unpredictably "two-group" all sorts of reloads into groups of 1/2 MOA or less, but almost an inch apart at 2:00 and 8:00, might have been because one/one pair of its lug was "high" at 5:00 and 11:00. Factory was only 3/4 MOA or so, but a rocking bolthead could have been leaving the heads out of square, and high spots on the reloads might have been throwing the bullets out into those two groups.

My M1A not being good for much better than 1.3 MOA (yeah, bedded, gas system shimmed, cheap match barrel and all that) makes it ill-suited for testing of this theory, and quantifying effects. I tried once but found only a hint of vertical if high spots are aligned at 12:00.

I'd love for someone with a 3/4-MOA or better M1A to test this out.

 

[Grump]

Re: NON-Square Casehead Effects on M1A/M14 Accuracy?

BTTT just in case another set of eyes on the Board tonight might now anything.