What is runout?

[bucksnort06]

I hear about this alot but not too sure exaclty what this is? Could somebody fill me in. If it can help me be a better reloader than I'd be interested of course. Thanks.

[skipper]

It can either be two circles (shoulder and neck) not rotating around the same axis or it can be a flat spot. It can even be a yaw from the centerline axis of the bore. Take a look at the drawing and see if it helps you understand.



The top drawing shows that the cartridge is not aligned with the axis of the bore. In order for a rifle to have peak accuracy everything needs to be aligned to the axis of the bore. The alignment could be either perpendicular or parallel but it needs to be precise. A good gunsmith can true up your rifle so everything is plumb and square. Only you can true up your ammo.

The drawing with two circles graphically represents what run out can look like between the case shoulder and neck. The example is extreme, I'll grant you, but it does illustrate what we are talking about. Everything needs to be aligned with the bore axis and perfectly rotating around the same axis. The other part of the equation is that the bullet must also be aligned with the same axis. Misalignment will show up on your concentricity gauge.

[bucksnort06]

Skipper,
thank you. that makes sense to me now.

[acloco]

(and this is a WEAK attempt at humor)


When you are 'splainin' to yer parents why you are two hours late....you just tell them you runout a gas.

That is a runout.

[OldTurtle]

Skipper, that was a good explanation and leads me to a secondary question, since I've never given runout a lot of concern.

If I'm using new or previously fired brass and after FL sizing, trimming, etc., I notice a problem with alignment when the bullet is seated, will this correct itself by firing the cartridge and resizing it ??, or is it a wasted case ??

[skipper]

Runout can be corrected. The case is not a lost cause. You need to find out where, in your process, the runout is being introduced. Take new brass, for example. After you full length size it, check the runout. Your full length die might be the culprit. Check again after each step of the process where force is being exerted to the case body, shoulder or neck. Then check right after you fire the brass, you might find out that your chamber is the culprit. Monitor each step and you will soon find out where it's coming from. After you find where it's coming from you can correct the problem.

Remember that every time you pull the trigger, the case will re-take the shape of your chamber. That's why I shoot the dented cases in a bag of new ammo as sighters. Once fired, there are just the same as the rest.

[WHISTLEPIG]

Skipper is correct and I will add that case wall thickness and neck wall thickness variations are a constant and are a factor in run out. The variation in case neck thickness is never taken away without turning the neck to a consistent dimension. The inconsistency simply moves from the exterior of the neck to the interior when the neck is fired or sized, and conversely to the exterior of the neck when a expander ball is dragged through it or a bullet is seated. If the neck on a case shows .005 thickness variation, it will always have at least .005 runout.

[OldTurtle]

After reading the initial post/answers, I looked at some new brass and FL sized them and ran then through my case trimmer and noticed that some were showing a very slight amount of trim on one side of the neck and not the other. That leads me to believe the initial size is not constant and was wondering if this will straighten out after firing.

I'm using Forster Benchrest FL dies and a Forster press. So far it seems to be giving me accurate loads and substantial neck tension for my ARs.

My case prep, after firing, indicates all my trimmed brass to be pretty consistent in case neck thickness, though I don't have the calipers to measure it adequately. When I roll a few on a mirror, they seem straight.

[Hawkeye Joe]

Old Turtle
Pick up a set of cheapo tube Mics to measure the neck thickness. Calipers are not suited for this job especially if the inside and outside are not parallel or concave. Here's my cheap set with a modified anvil and my Runout checker.