When is a chamber "non-concentric"?

[moosehunter71]

Hi there,

I have a factory M700 XCR chambered in the .300 WSM. I recently checked the neck runout on a sample of 10, once fired, fireformed WW brass (i.e., unsized). The average was .0023 (+/-.0002") using an RCBS Casemaster.

I have read on a variety of posts on other forums that neck runout in excess of .001" on fireformed brass would be indicative of a problem with my chamber or bore. However, in an article in the archives of 6mmBR (I think it was "Case Preparation from A-Z" by A. Gottfredson (s.p.?)), it states that one can expect from .003"-.005" neck runout on fireformed brass - and that this value may be improved after neck sizing.

My question is "Is my chamber non-concentric" or not (I have been measuring at the midpoint fo the neck)? OR what is a maximally acceptable amount of chamber non-concentricity - my goal is modest (I think) - I want a hunting rifle that will consistent should MOA. The answer may help me save $$$ on buying equipment that may not help because my rifle may be the problem (i.e., Redding bushing die, competition bullet seater, etc.).

In case it it is relevant

Accuracy is acceptable for my chosen load (1.09" for the 62.5 g H4350 - not the most accurate load, but it is for sure a safe load - shooting 180g TTSXx), and the average for 20, 3-shot groups using IMR4350 and H4350 is .97" (Standard Deviation = .38"; Max group size = 1.81; Smallest group = .44" with these powders).

 

[watercam]

Bear in mind that a total run out of .002" means that the case would be misaligned to the bore by half that (.001"). Out of a bore diameter of .308" we are talking about very small off set. If you outside trim your brass so that it cleans up around 60-80% you will have made your brass as concentric as needed without leaving the neck walls excessively thin. That should reduce any out of alignment to a minimum. Try that and then start shooting at longer ranges - I'll bet you will be pleased with your longer range accuracy.

 

[BBTC48]

A good smith can "hardcast" your chamber (including the lead and lands) then pull the cast and measure it all for you...a casting of your chamber. At issue is how well the chamber matches up with your bore. You'll also know exactly how much throat you actually have. The materials to do this are not expesive but without experience it may be a hassle. Chambers cut with "floating" reamers are notorious for mis-aligned chambers. This can be cured by setting the barrel back with a truely aligned chamber re-cut. Sorry, can't help with what's acceptable and what's not. Food for thought.

 

[BBTC48]

A quick PS...remember, you aren't shooting benchrest target bullets, you are shooting a solid copper hunting bullet. Also, neck tension consistancy is waaay important for those tiny groups. Maybe extra steps or care in neck brass prep could improve results. That could include identical case lengths. My best ammo is always the handful that I obsessed over in the brass prep areas. Also, I have tons of experience with Barnes and you could just jump 'em another .010" and shrink groups a bunch. Remember that they always say to jump their stuff a ton compared to what we all usually do with other brands of bullets. I just got into their new 130gr 6.5mm TSX and the groups were quite good when I jumped 'em .040" !! Good luck.

 

[Clark]

The eccentricity of ammo tracks linearly from .000 to .004" on the ammo to 0" to 2" group change, and the chamber bends the ammo straight beyond that. That is per the paper 50 years ago by the NRA, the Army, and A. A. ABBATIELLO, "Gauging Bullet Tilt" .

It is mistake, I fell for it, to think the same first order relationship exists between the chamber eccentricity and groups.

The reason is that the rotational orientation of the eccentricity of the ammo is random, but the chamber orientation is constant.

The centrifugal force flinging the bullet off course from random orientation throws the bullets randomly about the target, while the chamber eccentricity throw the bullets off course into the same small group.

 

[squirrelduster]

I think you may have your decimal point in the wrong place. The RCBS case master has a standard dial reading out in .001 increments so you are probably reading .023.
If this is the case you have a problem.
I have a friend that recently sold a 700 with a nonconcentric chamber his groups were around an inch and a half at 100 yards. He bought a new Savage 300 WSM and the concentricity issue went away and the groups are around a half inch with the same components.

 

[moosehunter71]

Thanks squirrelduster,

Out of curiousity, do you know by how much his concentricity was off (I am assuming that, like me, he measured the neck runout on firefored cases).

I actually remeasured 25 cases and found the average neck runout of the once fired, unsized cases to be .0021" (+/- .0002"). You are correct - the case master's smallest graduations are .001". However, I figure that I can accurately estimate the the last/10 thousandth decimal place to plus or minus .0002" accuracy - hence the .0021" value (+/- .0002").

Thanks

 

[21shooter]

Moosehunter, A chamber is non-concentric when the indicator moves off a set zero, within the entire lingth of the chamber.
John