Concentricity impact on accuracy

[BoydAllen]

Being able to see the differences that small changes may make, requires consistent attention to other details, or they get lost in the wind...literally. When was the last time that you saw someone who was not a benchrest competitor fine tuning his loads by loading at the range, or using wind flags? One local club that I belong to, has had benches that are so wobbly that it is a real feat to shoot well from them. We have begun to fix that. Another thing that I see is poor rest and bag setups. Few shooters understand parallax. Is it any wonder that some cannot see any difference between straight and crooked ammo? I do, and if you look in a book published by the NRA called Handloading ( I believe.) you will find an article that has a graph that shows a correlation between the concentricity of military ball and match ammo, that was written a long time ago. IMO concentricity may be more important for loose chambers than for tight ones. In a tight neck chamber, when bullets are seated into the rifling, there is a straightening effect that I have measured. The most striking example of the effect on loose chambers was when I discovered the secret to smaller groups with a scoped Enfield .303 was to size the brass in a manner that produced more concentric ammo. The difference in accuracy was substantial. Bullets were jumped.

 

[chkunz]

T-Rex,
Since few of us have the means to do real scientific tests we do the best that we can with what we have, and over time if our accuracy improves, perhaps we have made some observations and changes that are valid. I have a good friend that has an engineering background that tends to go off onto discourses abut what is needed to do proper testing. Of course he has never done any to that standard. From time to time I have helped him improve his results with some of what I have picked up unscientifically, by repeated trial and error. There is a part of the shooting sports that is known as short range benchrest. Perhaps you are familiar with it. I would venture that none of the advances that it has produced in the last sixty years or so have come through true scientific research.
Boyd

Sometime in the 1960s a man named A. A. Abbatiello wrote an article that was published in "The American Rifleman" where he presented data showing the effect of bullet "tilt" (run out) on accuracy and the effect on group size for a specific rifle/ammo combination. It was impressive and it became a national pastime to measure bullet run out and pursue ways of reducing it. Then someone else did some testing (maybe Creightpn Audette but I can not remember for sure) where he shot groups prone at 300 yards from an XTC match rifle of known accuracy. Some groups were shot with a known accurate load without regard to run out, some groups were shot with selected low, near zero run out and some groups were shot with ammo that the bullets were intentionally misaligned for significant run out. The results did not show any statistically significant difference in the accuracy of straight ammo and crooked ammo. As a result there was much less interest in the subject by many folks. it may be that for more demanding accuracy like F Class it is significant but I have not seen where anyone has done any testing and reported the results. I think this is what the OP is asking, does anyone have any meaningful data on this subject? I reported on what I did with the M14 on this subject and there was another reply as well and at that level of accuracy we could not see any correlation. I think it is feasible to test the effect of run out on higher accuracy configurations and it would be good to hear from someone who has done this, again I think that is the OP's question.

 

[BoydAllen]

Have you read Audette's article that goes into the effects of variation in case wall thickness? In my experience, cases that are more uniform in this respect, produce straighter ammo, which shoots better.

 

[jlow]

Being able to see the differences that small changes may make, requires consistent attention to other details, or they get lost in the wind...literally. When was the last time that you saw someone who was not a benchrest competitor fine tuning his loads by loading at the range, or using wind flags? One local club that I belong to, has had benches that are so wobbly that it is a real feat to shoot well from them. We have begun to fix that. Another thing that I see is poor rest and bag setups. Few shooters understand parallax. Is it any wonder that some cannot see any difference between straight and crooked ammo? I do, and if you look in a book published by the NRA called Handloading ( I believe.) you will find an article that has a graph that shows a correlation between the concentricity of military ball and match ammo, that was written a long time ago. IMO concentricity may be more important for loose chambers than for tight ones. In a tight neck chamber, when bullets are seated into the rifling, there is a straightening effect that I have measured. The most striking example of the effect on loose chambers was when I discovered the secret to smaller groups with a scoped Enfield .303 was to size the brass in a manner that produced more concentric ammo. The difference in accuracy was substantial. Bullets were jumped.

I completely agree with what Boyd said. It is extremely hard to measure these small differences, not to say that they don’t make a difference.

If one don’t think this is true, one does not have to look any further than the age old question of “does the weight of brass case correlates to their volume?” ;D In this MUCH easier example where both can be measured right on the bench and wind is NOT a factor, we can get uncountable number of pages of debate. :

 

[chkunz]

Have you read Audette's article that goes into the effects of variation in case wall thickness? In my experience, cases that are more uniform in this respect, produce straighter ammo, which shoots better.

Yes, I have read it. He wrote a five part series titled "Testing Rifles and Ammunition" that was published in Precision Shooting, October 1993 and in one part he investigated the effects of case wall thickness on case head squareness and then showed the effect on the target. I think there were different results for different bolt lug configurations but it has been awhile since I read the article. Some of the information he presents in this five part series has been published in other sources. All good information.

 

[mikecr]

Anyone doing any testing won't help.

"Just because we can not know everything does not mean that we can not know anything."

And broad declarations usually represent narrow understandings.

I'm suggesting this subject currently leads to an abstract, and this is important to recognize..
So for as many who say it matters, there will be as many who say it don't.
If it matters at some point, nobody knows when.
If it doesn't ever matter, nobody knows why.
That's the summary of it.

 

[BoydAllen]

I have seen rounds straightened by chambering in a tight neck chamber, when bullets are seated into the rifling. I have not found this to be the case in factory chambers when bullets were jumped. In the latter all but the most extreme cases probably were lost in a myriad of other imperfections in the system, BUT even so, I have personally seen cases where straighter reloads resulted in demonstrably better accuracy. Straighter ammo will only fix problems cause by lack of straightness. Commonly, I see shooters working with rifles in such a way that these sorts of distinctions are probably totally meaningless.

 

[JarheadAZ]

Boyd;

When you respond, I always pay attention. I am prepared to revisit my own testing on this subject.

Most of my shooting history has been devoted to getting factory rifles to shoot more precisely. All of my rifles have SAAMI chambers, so I suspect that actually gauging concentricity might result in some ambiguity. So I'm going to try something different.

I the vein of seeing if the ammunition can be straightened by the chamber neck, I'm going to try something outside the box.

My F/L die is adjusted a turn or so high, resulting in a neck resizing process that leaves the bottom 1/16" or so of the neck, just above the shoulder junction, unsized. I do this because I neck down 7-08 brass to .260 size, and I'm trying to avoid driving any 'donutty' imperfections to the inside of the case neck. Must be working, or no donut exists, because I seat bullets without any noticeable ramping up of seating force.

When these cases are completed rounds, a noticeable increase in neck diameter exists just above the shoulder junction region, yet the cartridges chamber without any fuss. I seat to jump, by whatever the difference is between contact and magazine feeding length. I'm guessing that the 'step' assists somewhat in centering the neck in the chamber, perhaps in a manner similar to how a tight neck works, but without any issues related to inadequate clearance for bullet release. It might add some accuracy benefit to a 'no turn neck'.

Obviously a bump is occasionally needed, so I use an otherwise unused .308 die as a bump-only die.

I think that with the raised neck 'step', a condition similar to the tight neck 'straightening' process can be recreated in the SAAMI chamber, and that with that 'step' eliminated, concentricity will be unaffected during chambering, and a different outcome on the target can be achieved (maybe).

And maybe the Moon really is made of green cheese...

...And there is also the issue of my own clearly imperfect shooting skills.

Anyway, I plan to do this test sometime in the coming year. If it works, maybe this process can have some benefit for SAAMI chamber shooters.

Greg

 

[rap36case]

I have a gauge with an way to 'bump' them into being more concentric. Does not seem to make any difference.

What seems to work most are excellent dies, reloading practice and tossing the brass that produce flyers. At least set them aside for further testing.

The book on 'Reloading for Competition' and the excellent articles here help.

 

[BoydAllen]

When you neck down a larger caliber to a smaller one, and leave the shoulder location alone, you can do a couple of things to keep doughnuts at bay. First of all find the die setting on your .260 die that gives you some feel as you chamber a newly necked down case. This should keep the case against the bolt face for that first firing. Use a fairly stiff load for that firing to put the shoulder as close to max as possible. Once you have done that, you will be in better shape for doughnuts than if you had started with a factory .260 case, since the back of the old 7mm neck will have become part of the shoulder. You will not have heavy shoulder brass right next to the back of the neck. The best way to have concentric loads with unturned brass, and factory chambers (that I have seen) is to do two step sizing, using a Lee collet die first, followed by a body die adjusted for proper shoulder bump. Over the years I have had a lot of positive feedback on this. Just remember, with the collet die, neck tension is controlled by mandrel diameter, and Lee will make you different sizes.

 

[JarheadAZ]

Thank you, Boyd.

I had already considered the first part (which is why I don't neck up).

I will take second part under serious advisement after I get through turning my thought experiment into a practical experiment. Be aware that incorporating additional specialized implements runs counter to my simplify..., simplify... process. Cutting corners is not my middle name, but I'm considering it...

I do a lot of thought experiments, trying to think things through and mentally visualize actions and consequences.

One thing at a time, I'm getting old and slower is better. What seems slow is often just the consequence of more thorough previous thought.

I don't fireform, I prep (including F/L resizing) and load my new brass with the previous best load and do load verification; but I also set the brass up to close with some definite drag. It's better than nothing, even if just barely. Once a roughly significant sample (25-30 or so) proves itself, the rest is used for the intended purpose. When it proves less adequate, I have the RCBS Collet puller...

Remember, I owe most of what I do to Benchrest shooters, but I am not one myself by conscious choice.

Greg Langelius