what is bullet runout


July 4, 2008, 11:37 AM
Can anyone explain this to me?

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July 4, 2008, 11:45 AM
Bullet runout is the deviation from being perfectly round. If everything is perfectly aligned, then when the loaded round is spun, there would be no deviation when the bullet is measured from the side. The runout is the amount of measurement the bullet "wobbles" in alignment with the case neck when spun slowly in a device for measuring the "wobble". If you envision two objects joined end to end, and then spun in your fingers, the amount of "wobble" between the two objects would be the runout.

I don't know if this is a clear explanation or not, but I hope it helps.


July 4, 2008, 11:57 AM
Sure. Bullet runnout is how much the bullet is out of line with the center line of a loaded round. It is measured with an instrument like this one from RCBS;


The RCBS tool uses a dial indicator to read off the bullet while the case is turned in two "V" blocks. Any mis-alignment shows up as wobble on the dial indicator.

Theory is that bullets held in perfect alignment will enter the rifling perfectly square enhancing accuracy/group size. You can also check to see if your cases are being sized properly, without being bent or out-of-round. This is done BEFORE priming, powder or seating a bullet. The final check tells you how well the seating die is working.

July 4, 2008, 12:11 PM
What is run-out. or as it is more correctly called concentricity. As shooters have become more demanding in their search for accuracy, specialized reloading equipment has come into the market. It is well known that bullet run-out, or concentricity, is a major factor in producing accu-rate ammunition.

In years past, the accepted practice for checking concentricity of a handloaded round was to roll the cartridge across a flat surface, such as glass, and note any wobble at the bullet’s tip. This was a fast way of culling obviously defective cartridges, but will fail to isolate those with less obvious run-out problems. Add to this the fact that run-out problems can be caused by not just an improperly seated bullet, but by the case itself, and the limitations of this approach become unacceptable.

Today, handloaders have several options that are capable of measuring concentricity to .001" or less. Most operate on some variation of the same principle. A loaded cartridge is mounted in the unit, normally supported by a “V” block arrangement at the case head and bullet ogive. The cartridge is rotated slowly, while a dial indicator bears on the area of the cartridge being checked. Any concentricity problems are not only immediately visible, but measurable on the dial indicator.

One of the first commercial models, which is still readily available and quite popular, was the Forster/Bonanza Co-Ax Indicator. Recent entries in the concentricity gauge market include models by Sinclair International, NECO, and RCBS. Some models, such as the NECO Gauge and RCBS Case Master, are capable of not only measuring bullet run-out, but case neck variation, wall thickness and concentricity as well. Given the accuracy obtainable, particularly from many of today’s bolt action rifles, using a concentricity gauge to get the last bit of accuracy out of handloaded ammunition makes perfect sense.

July 4, 2008, 12:16 PM
And here I was, all this time, thinking that "bullet runout" was that thing that told you it was time to pick up your brass and get back to the press! :neener:

38 Super Auto
July 4, 2008, 01:53 PM
Very good descriptions.

I wanted to add that, while in the barrel, the bullet rotates about its center of form, but once the bullet leaves the barrel, it rotates about it's center of gravity.

In the case of a perfect bullet, the dimensional center and center of mass are equal. For a real life bullet, these two are slightly different, or may be vastly different due to voiding, sizing, improper fill out, lube anomalies. etc.

The Hornady Handbook of Cartridge Reloading states that concentricity needs to be better than 0.5mil for best results. YMMV :uhoh:

July 4, 2008, 02:13 PM
not to hijack the thread but since we are talking about bullet runout. would turning the case necks fix the bullet runout problem?

July 4, 2008, 03:06 PM
Variations in neck thickness are one source of runout.
Variations in the bullet profile are another )not perfectly round).

Another cause is a bullet not seated straight into the case.
Remember that bullet to neck fit is an interference fit.
We make the neck smaller and force the bullet in, leaving the neck to stretch and hold tightly.
If the bullet is not held straight it can enter with its axis out of parallel to the case neck and end up not being perfectly straight.

July 4, 2008, 03:22 PM
Thank you all for your replies. The issue is much clearer now.

July 4, 2008, 03:45 PM
And here I was, all this time, thinking that "bullet runout" was that thing that told you it was time to pick up your brass and get back to the press!


July 4, 2008, 05:09 PM
Speak english!

The Hornady Handbook of Cartridge Reloading states that concentricacy needs to be better than 0.5mil for best results.

What the @#^* is 0.5 mil?

In my experience, anything over .003- .004 is too far out to be expected to be accurate. Now we are speaking mainly of group size here. Accuracy can also be seen as hitting any give point, like a bullseye. That to me means adjusting your sights to hit where you want to. Of course a smaller group ability means you're more likely to hit a precise point.

I'm going to go take some pics of my case master while measuring some .223 I just loaded. That should clear up some questions.

July 4, 2008, 10:39 PM
As promised.

1. Here I'm measuring overall runnout or concentricity. In this pic I have the dial indicator on zero.


2. This is the max out of round that this nosler 69 match HPBT has. .002 is good!


3. Now I'm measuring just how straight the sized case neck is. You could also go up on the case body to measure there. This tells you how well your FL sizer is working.


4. The neck was at .001, also good run out.


5. The RCBS case master has a means of measuring case neck thickness, and see if the thickness varies. This pic doesn't show up too good, the tool is black and I'm depending on the built in flash of the digital camera. It was only .001 difference quite good. Oh-those are R-P cases.


6. Here's the stud the case neck rests on when checking neck thickness and concentricity.


7. the skinny rod that can be seen in several of the above pics is for checking for the groove found inside cases that are going to have case head separation on the next firing. Here I put the case on that rod to feel for the crack inside.


I hope this clears up what runnout is. I find the case master to be well worth the $, it's well made and works well. Not saying the others don't, just that this tool does more for the bucks.

July 5, 2008, 10:32 AM
What the @#^* is 0.5 mil?

0.0005 inch.
A 'mil' is often used in machining for 0.001 (one thousandth) of an inch).

July 5, 2008, 11:20 AM
Thanks brickeyee, but what 38 SA is talking about is the bullet, or projectile, not the assembled round runnout. .0005 is a very small runnout for the concentricity of a loaded bullet in a brass case. I don't think even the bechrest guys with their arbor presses can achieve that.

The Hornady Handbook of Cartridge Reloading states that concentricity needs to be better than 0.5mil for best results. YMMV

I thought that 0.5 mil meant millimeter! Again, I grew up-went to school over 40 years ago. I never converted to metric, never will.

I just went to see what a half millimeter looked like. By setting my digital caliper to metric, wow you could throw your hat through that gap!:what: So 1 mil is .001? I'll have to remember that.

July 5, 2008, 11:48 AM
Excellent photos and description Snuffy. I did not have a clue what .5 mil was either. ;)

I use this little NECO set up.

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