Concentricity, how I fixed mine

[Bart B.]

Last box I had of Rem BR brass, primer list on box is small rifle primer 308 and look like flash hole been drilled out. What else you do to those cases?

That's 2 views 180 degrees apart of the same case.

I only uniformed pocket depth after first firing. Nothing else.

 

[Shadow9mm]

So I don't check runout. I have read a lot of stuff on it. My question would be. Does it really make a difference. If so how much are you getting for the extra work.

Let's say a 5 shot groups, one with runout of 0.010, one at 0.005, and one at 0.001. How much difference will there be at the target?

 

[hounddawg]

Depends on the rifle and the skills of the person shooting. Go buy 100 rounds of factory ammo, sort them into groups by runout then shoot them. See if it matters in your rifle with you behind the trigger. It may just get lost in the noise of wind and technique or you may see a difference.

 

[Shadow9mm]

Depends on the rifle and the skills of the person shooting. Go buy 100 rounds of factory ammo, sort them into groups by runout then shoot them. See if it matters in your rifle with you behind the trigger. It may just get lost in the noise of wind and technique or you may see a difference.

Just where exactly am I supposed to get 100rnds of factory ammo? Even if I could find it there is no guarantee there woukd be any reasonable spread of concentricity within the batch. Also, I don't have a concentricity gauge.

My apologies. I expected that you might have some data to share to show the difference it made to justify the time, effort and equipment you put into "fixing it". I have yet to see concrete evidence that it makes any discernable difference.

 

[jetinteriorguy]

Delete: Didn’t realize I’m being somewhat repetitive with a previous post.

 

[Unclenick]

Let's say a 5 shot groups, one with runout of 0.010, one at 0.005, and one at 0.001. How much difference will there be at the target?

It depends on the bullet design. How far the center of gravity is from the geometric center of the bullet's bearing surface determines how far each thousandth of bullet tilt will move the bullet center of gravity away from the bore axis. If a double-ended wadcutter, there is zero difference, so tilt doesn't affect them. For long bullets where the center of gravity is further forward of the bearing surface, it is significant. A. A. Abbatiello's old article in the Rifleman in which he measured runout and fired 829 rounds from forty-two lots of National Match 30-06 loaded with the 173-grain, seven caliber tangent ogive boattail bullet with long boattail and 0.89 caliber bearing surface. He found the group radius increased about ⅛ moa for each 0.001" of bullet tip tilt (try saying those three words ten times really fast) up to 0.0045" of tilt, beyond which additional tilt made no difference because the bullet would straightened greater amounts down to 0.0045" of tilt in the bore. In other words, the the diameter of the group could increase up to 1⅛ moa.

Abbatiello references a similar study with similar results by George L. Jacobsen of Frankford Arsenal, published on page 20 of January 1960 issue of American Rifleman.

Shooting a 68-grain 6 mm flat base bullet, Harold Vaugn repeated the experiment but got only about 40% of the radial dispersion Abbatiello got. However, the bullet he used has a longer bearing surface (in calibers) than the 173 does, so it will straighten a bullet more. It's just important to note how dependent on bullet shape the effect is.

Abbatiello said between 20% and 30% of bullets in the boxes were tilten by 0.003" or more. If you read the two pages on Vaughn's experiment, you will find that by always pointing an equal bullet tilt in the same direction in the chamber, the error and its direction remains the same. So if you have commercial ammo and sort it by runout, you can intentionally shoot groups oriented 180 degrees apart in the chamber and see the worst case effect of random orientation. Then shoot the same number of rounds all tipped the same way, and you will get a group the best size you can expect from the ammo. Abbatiello noted that if all he did was mark the high side of all his cases and always oriented them the same way in the chamber, he cut groups roughly in half as compared to letting them be randomly oriented in the chamber.

 

[hounddawg]

Only study that counts is the one a person does themself. Every situation is different and one person may change something and see an improvement for while someone else could do the same thing and see no change becasue that person may not need that aspect improved. Precision shooting is a triad of shooter, rifle, and ammo and we all have to figure out which leg of the triad needs bolstering.

Best advice I have ever read on precision shooting

 

[Shadow9mm]

It depends on the bullet design. How far the center of gravity is from the geometric center of the bullet's bearing surface determines how far each thousandth of bullet tilt will move the bullet center of gravity away from the bore axis. If a double-ended wadcutter, there is zero difference, so tilt doesn't affect them. For long bullets where the center of gravity is further forward of the bearing surface, it is significant. A. A. Abbatiello's old article in the Rifleman in which he measured runout and fired 829 rounds from forty-two lots of National Match 30-06 loaded with the 173-grain, seven caliber tangent ogive boattail bullet with long boattail and 0.89 caliber bearing surface. He found the group radius increased about ⅛ moa for each 0.001" of bullet tip tilt (try saying those three words ten times really fast) up to 0.0045" of tilt, beyond which additional tilt made no difference because the bullet would straightened greater amounts down to 0.0045" of tilt in the bore. In other words, the the diameter of the group could increase up to 1⅛ moa.

Abbatiello references a similar study with similar results by George L. Jacobsen of Frankford Arsenal, published on page 20 of January 1960 issue of American Rifleman.

Shooting a 68-grain 6 mm flat base bullet, Harold Vaugn repeated the experiment but got only about 40% of the radial dispersion Abbatiello got. However, the bullet he used has a longer bearing surface (in calibers) than the 173 does, so it will straighten a bullet more. It's just important to note how dependent on bullet shape the effect is.

Abbatiello said between 20% and 30% of bullets in the boxes were tilten by 0.003" or more. If you read the two pages on Vaughn's experiment, you will find that by always pointing an equal bullet tilt in the same direction in the chamber, the error and its direction remains the same. So if you have commercial ammo and sort it by runout, you can intentionally shoot groups oriented 180 degrees apart in the chamber and see the worst case effect of random orientation. Then shoot the same number of rounds all tipped the same way, and you will get a group the best size you can expect from the ammo. Abbatiello noted that if all he did was mark the high side of all his cases and always oriented them the same way in the chamber, he cut groups roughly in half as compared to letting them be randomly oriented in the chamber.

Everyone needs and Uncle like you sir.

So if I am understanding correctly. A run out of 0.0045 or more could essentially add a 1/2moa to a group if I am understanding correctly.

Very interesting. Looks like I will be adding a concentricity gauge to my collection of tools in the near future. In this case, I cant tell if it needs to be fixed without being able to measure it.