At what point does cartridge runout erode accuracy?

[Mulligan]

I have searched the web, cannot find a good study or really any solid information on the relationship between straight ammo and accuracy.
Is there a correlation between small groups and really straight ammo?
Does building concentric ammo help with accuracy? If so, how much matters?
CW

 

[Stan Taylor]

Hold that thought ,give me a little while, I got to go down the road and stock up on some popcorn, I’m interested in hearing this.

 

[jepp2]

Does building concentric ammo help with accuracy? If so, how much matters?

I looked for the same answer. Best I can seem to come up with is if your ammo bullet TIR is over 0.005" it might affect accuracy some unknown amount. If less than that, probably not.

I spent a couple of years working on reducing my case neck and bullet TIR. I now have case necks less than 0.001" TIR always. Bullet TIR is running 0.002-0.003" at maximum. Can't say it has improved my group sizes at all. But I did learn about a lot of things that induce runout.

 

[bozo699]

Mulligan,
I used to sort ammo for perfect concentricity and or straightened it but I never saw any difference on target however because I’m ocd about it I still sort my match rounds with the least runout. With good components and equipment and loading practices I have very little runout anyway
Wayne

 

[Mulligan]

Hold that thought ,give me a little while, I got to go down the road and stock up on some popcorn, I’m interested in hearing this.

We are waiting, did you get your popcorn?
CW

 

[Mulligan]

Hopefully someone has tested it and will share their findings
CW

 

[tmwinds]

I can’t determine if straight ammo shoots better but I did determine that zero runout gave me a “fuzzy feeling “.

 

[JimSC]

No idea how much affects accuracy but if you are making precision ammo with consistent neck tension odds are it will be straight anyway. I just loaded 25 rounds of .308 for load testing and the largest runout I had was .002 on a handful, the majority was .001 or less. That was made doing the sizing and seating using a Lee Turret press, which shows me you don't need a 1500 dollar press to get it done right. I used a Whidden FL sizer with no expander, 21st Century expanding mandrel and a Forster seating die. Nothing fancy

 

[chkunz]

I did the test with the M14 at 600 yards sometime in the 1990s and documented the results. I have reported on this a couple of times on this forum so you may be able to find the details under my name.

For a Master class shooter with the M14 at 600 yards there was no statically significant difference between straight ammo and crooked ammo at 600 yards. But if I have a choice between straight ammo and crooked ammo I will choose straight ammo.

 

[KMart]

Ignorance is bliss. I don't have tooling to measure runout. One less thing to worry about.

 

[PigButtons]

I can't put my hands on it but I saw a study from one of the high end rimfire manufacturers ( so understand there was bias in the study ), that went into this extensively. The main take away was that if the bullet is not concentric when it first engages the rifling it will remain slightly misaligned while in the barrel. Upon exiting, the nose of the bullet will be making tiny spirals. The good part: the smaller the spirals the less "LIKELIHOOD" of a flyer. The bad part: no numeric value could be placed on the amount of error that would be introduced, therefore no cause and effect relationship could be established.

 

[Straightshooter1]

I have searched the web, cannot find a good study or really any solid information on the relationship between straight ammo and accuracy.
Is there a correlation between small groups and really straight ammo?
Does building concentric ammo help with accuracy? If so, how much matters?
CW

Though I don't know of any direct study on this, some years ago there was an article Secrets of the Houston Warehouse that pretty much answered this question for me, so I'm attaching a copy here if you're so inclined to read far enough into the 9 pages:

PS: From what is written, it was important to Virgil to keep the bullet's axis in line with the bore's axis and why where felt the bullet should be into or touching the lands, which goes to maintaining concentricity. But note the the minute details he goes to to get his guns to shoot in the zeros.

 

[DiffEQ]

Have a beer or three before you shoot said ammo. It'll shoot right.

 

[Coyotefurharvester]

Doesn't the bullet just squeeze through the barrel and go where the barrel is pointing?lol

 

[ballisticxlr]

If someone could help me out with a couple gauges and a measuring fixture I could borrow for this purpose, I will conduct this experiment across 2 rifles of statistically known performance.

I figure we'd need to do bullet runout independent of the case (bullet cockeyed in the neck) and runout in the case independent of the bullet (bullet cockeyed into the throat by a cocked case) and a combined set with runout of a cockeyed case and a cockeyed bullet in at minimum 16 variations (32 variations would be better) to catch bullet runout at different relative direction to the case runout.

 

[XTR]

There are a lot of potential variables. One is long throated barrels with tight freebores and heavy for class high BC bullets. The combo doesn’t allow much for runout, they will straighten the bullet in the bore when you load them. In short flat based bullets that short range BR shooters use, maybe that is a different animal.

About 10 yrs ago an old BR shooter was at the range with me, this guy was a forever tinkerer/tester. He told me that in his testing that he could not see any difference on the targets with less than .009 runout. I haven’t checked runout since then. I shoot F class, my worst bullets have less than .005 and usually had .003 or less. Most of us should be in the same boat with good equip and technique. There are enough other things I can spend my time on.

 

[ballisticxlr]

Doesn't the bullet just squeeze through the barrel and go where the barrel is pointing?lol

Not entirely. The bullet can enter the throat either concentrically or not. If it's concentric; and so is everything else in the system, then the lands will all begin to engrave the jacket at the same time and the forces are evenly distributed. If it enters out of kilter then the engraving forces on one side "could" be different and the resultant swaging of the bullet to make it straight again will change its shape somewhat and could unbalance the mass distribution which could, because of rifling and high spin rates that come of it, cause an imbalance once the bullet is in free flight with essentially unpredictable consequences. On the whole though, I suspect that what XTR talks of, the straightening effect of the throat, will dominate.

 

[JFrank]

When it comes to this topic, I could not see any difference in my groups nor could I shoot the difference up to five thousand out, then I got distracted and started thinking about in bore yaw and how one may be tied to another if at all.
I really think you would need a rail gun to test with. Hint Hint @Mulligan

 

[Fuj]

The first barrel I had chambered with my SA284,, I did a casting
and was surprised how tight the the free bore measured and a
.290 length. Even with a decent Detroit lean, it gets squared
away by closing the bolt. Now for yinz running very little free
bore, Run out would be a concern.

 

[Stan Taylor]

There are a lot of potential variables. One is long throated barrels with tight freebores and heavy for class high BC bullets. The combo doesn’t allow much for runout, they will straighten the bullet in the bore when you load them. In short flat based bullets that short range BR shooters use, maybe that is a different animal.

About 10 yrs ago an old BR shooter was at the range with me, this guy was a forever tinkerer/tester. He told me that in his testing that he could not see any difference on the targets with less than .009 runout. I haven’t checked runout since then. I shoot F class, my worst bullets have less than .005 and usually had .003 or less. Most of us should be in the same boat with good equip and technique. There are enough other things I can spend my time on.

As of two minutes ago I did just what you just said my bullets are at .010 jammed I checked concentricity in which was less than .001 loaded that round an pulled it cut concentricity in half thanks.