How important is Concentricity?

[Sky Dog]

I'm always wanting to make my reloads more accurate. I see Hornaday has an alignment jig to correct bullet runout. It seems to me the bullet will straighten itself out as it goes down the barrel. Is this a legit concern or just a marketing ploy?

 

[Varminterror]

It seems to me the bullet will straighten itself out as it goes down the barrel.

It doesn't. Tangential ogives help, but there's no free lunch.

Eccentricity will result in the bullet contacting one portion of the leade before the rest, which tips the bullet slightly, and that yaw remains as the bullet travels down the barrel. The tip will scribe a helix as it travels down the barrel. Think about American Football - If a QB throws a tight spiral, it flies well. If they throw a "duck," it wobbles throughout the flight, and they lose range & accuracy.

The efficacy of the Hornady bullet alignment tool is marginal at best. It takes forever, and it only pushes on one part of the bullet at a time. The better investment is to make ammo which is more concentric from the jump.

 

[jmorris]

I built a few different gauges to measure runout in different ways and tested different amounts in different firearms, different dies and presses too.

I found the dies make a larger difference than what press they are in and what firearm/round combination makes a difference on end results.

On firearms that didn't seem to care, I noticed that a round that had run out, if chambered, not fired and removed to remeasure, was "fixed" by being chambered.

That's not the case everytime though and if you what to know how important it is in any particular combination, you have to test it.

 

[FN in MT]

I converted over to almost all REDDING rifle dies in the past ten yrs or so. In either my old RCBS Rockchucker press or my Bonanza co-ax, they provide minimal runout. At one time I had an array of assorted dies going back to the 1960's. Dies are FAR more precise NOW than they were back then.

If your rifle use is a whitetail at under 200 yds I'd not worry about runout. But if your game is LONG RANGE at small targets....WORRY.

 

[edwardware]

I wondered this exact wonder this past year, so I did an experiment!

I built a concentricity gauge and characterized a couple batches of ammo for my R700 SPS Tac (.223 bull barrel), benched and bagged. In a batch of 50 rounds I separated into 3 tranches, A: 0-3, B: 3-10, and C: 10-30; units are 0.01mm of runout, measured at the leading edge of the bullet's major diameter vs the case shoulder and case base.

I found. . . no difference whatsoever in group size! I shot the groups across 3 range trips, A, B, C, B, A then C, B, A, B, C, etc. I was a rigorous as possible within the constraint of 50 rounds (and I design test plans for a living).

I'm certain that concentricity effects accuracy, but I have proven to myself that with my equipment and my average group size of ~1 MOA, it's not measurable. After I've shrunk my groups down to ~0.75, I"ll rerun the experiment and see if I can measure the effect.

If you don't yet have a gauge, save your money and do your own experiment; take a batch of 50 identical rounds and separate them visually by rolling across a flat surface and looking for wobble. Shoot your 10 best and 10 worst, and see if you can measure a difference.

Edit: I also found that, just as Redding will tell you, the vast majority of runout is caused by brass (neck thickness and modulus variation) and the sizing die. The seating die doesn't seem to matter at all. This discovery cost me a $100 Redding seating die; it's wonderfully adjustable, precise, and repeatable, but the ammo it produces is no more concentric than what I make with my roughly-finished Lee seater.

 

[FN in MT]

Ed....Tested at 100 yds? Of course there will be little difference at close range. Try 300 or 500 yds. You WILL be surprised. Concentricity DOES matter at longer ranges. But for the guy who shoots his deer at 200 yds or gophers out to no more than 300 yds....the difference IS very small.

 

[edwardware]

Ed....Tested at 100 yds? Of course there will be little difference at close range. Try 300 or 500 yds. You WILL be surprised. Concentricity DOES matter at longer ranges. But for the guy who shoots his deer at 200 yds or gophers out to no more than 300 yds....the difference IS very small.

The dispersion due to eccentricity should scale well with distance (unlike BC variation that's not easily measurable at close range) so if it's not measurable over a ~1MOA group at 100 yards, I wouldn't expect it to be measurable over a ~1MOA group at 500 yards.

Without question, eccentricity degrade accuracy, but not enough for me to measure with my average ~1MOA groups. If I was a 0.5MOA shooter, I'll bet it would be matter.

 

[Sunray]

Most of the gadgets and gizmos sold these days are marketing. The seater die and expander button have more to do with concentricity than any gauge. So does using the right bullet. Hunting bullets, for example, are not held to the same roundness standard as a match bullet. Mind you, a hunting bullet doesn't need to be. Hunting accuracy isn't as precise as match accuracy.
Whole thing makes no difference in a hunting rifle anyway. Hunting rifle barrels aren't made to the same straightness standard as a match grade barrel either. In any case, for hunting, consistency is more important than group size.

 

[Nature Boy]

I believe run out has an affect on precision

I did a little test a while back showing the results on .308. Each target was 10 rounds



I use the Hornady concentricity tool to correct all my rounds to within .001 TIR

 

[edwardware]

I believe run out has an affect of precision

I did a little test a while back showing the results on .308. Each target was 10 rounds

View attachment 246047

I use the Hornady concentricity tool to correct all my rounds to within .001 TIR

I like it; you did a test and collected real data!