Reloading Basics: Bullet Concentricity

With so many variables in the reloading process, and the seemingly endless pursuit of reloaded ammunition that’s ‘perfect’, the more of them that you can measure and correct the better. Ideally, you want to make every round the same as the one before it, and the one after it. Concentricity and bullet runout are variables that are well worth some extra attention, as they can have a major effect on the consistency of your ammunition.

Concentricity

The word concentricity is derived from ‘concentric circle’, the term used to describe circles that share a common centre point. When it comes to ammunition, it refers to having every element of a cartridge sharing the same centre, from the flash hole to the outside of the case, the neck, and the bullet. If all the components share this common centre, they will enter the bore of the rifle consistently and this will result in better results on the target. Ideally, the bullet should also be concentric with the bore of the gun.

Bullet runout

Bullet runout is the amount that a complete round of ammunition is out of line with its actual centreline, from the centre of the head of the case to the very tip of the bullet. Put more simply, bullet runout is the amount of deviation from perfect concentricity.

Bullet concentricity issues

.
If the case or the bullet is not correctly aligned then, when fired, the bullet will not enter the bore consistently. It is possible that the chamber pressure will force the bullet to align with the bore, but the bullet is likely to be damaged and over-stressed in the process, even if it does correct itself. If the bullet comes out of the case mouth crooked because it was seated crooked, it engages the rifling cocked slightly sideways and it must immediately align itself with the bore or be swaged into a new shape by the pressure behind it. Whatever happens, a rifle cartridge with excessive runout is not going to perform well.

Factors affecting concentricity

Concentricity, in terms of reloading, is affected by several factors. In terms of the case, we use them ‘as supplied’ and there is very little we can do to address any manufacturing variances. If the primer pocket or flash hole is off-centre, then there is little we can do, other than use a flash-hole uniforming tool to make them as consistent as possible. If the body, shoulder, or neck are not true, then fire-forming the cases is the best way to bring them all in line with each other, or at least in line with the chamber and bore of your gun. The thickness of the brass at the case mouth is also critical because thicker areas will effectively push the bullet over towards the thinner side during bullet seating.

Resizing cases is a significant factor in ‘standardising’ them before reloading, and it is important to do it consistently. The resizing die, either full-length or neck-only, must be installed in the press correctly and securely with no play or movement. A good fit between the shell holder and the press, and the shell holder and the case, is necessary to ensure that every case is held and resized exactly the same way. ‘Play’ in the press, particularly with turret presses, can also affect consistency and concentricity.

To improve concentricity during bullet seating, sit the bullet straight in the case mouth, seat it halfway into the case, back it out of the press, rotate the case 180º, and then seat the bullet the rest of the way. This significantly reduces bullet runout issues and addresses some of the tolerances in your press and die.

The way that you start bullets into the case is also critical. If you stand a bullet in the case mouth crooked and then raise it into the bullet seating die, chances are it will seat crooked. Dies with a side window, through which you introduce the bullet and allow the die to centre it over the case mouth, are the best way to avoid crooked bullet seating.

Measuring and correcting concentricity

There are several tools available that enable you to measure and correct bullet concentricity. The Hornady Lock-N-Load Concentricity Tool allows you to identify, measure, and significantly reduce bullet runout. It is also an ideal tool to demonstrate how such equipment works.

A round is inserted into the tool, where it is held securely between two concentric spindles. A smaller spindle, connected to a dial indicator, is then introduced until it comes into contact with the side of the bullet, as far forward on the bullet shank as possible, and without touching the ogive section. When the round is rotated, the needle on the dial spindle will move, indicating the amount of bullet runout.

On the opposite side of the bullet to the dial indicator, is a nylon-tipped thumbscrew that is used to push the bullet back into alignment and remove as much of the bullet runout as possible. After ‘straightening’ a cartridge, if you have reduced the total amount of runout to between 0.002” and 0.003”, you have probably achieved the best accuracy and consistency you can get with home-loaded ammunition.

Conclusion

For most reloaders, the process of measuring and correcting concentricity is probably a step too far, and they do not have a tool to do the job on their bench. However, this might be a bit short-sighted, as out of all the more ‘advanced’ reloading processes, this is the one that does make a real difference to the performance of reloaded ammunition. Even factory ammunition can show some degree of bullet runout, and you can correct it just as easily as you can with reloaded ammunition. Having addressed the runout on a few batches of ammunition, and after seeing more consistent results on the target, all my long-range rifle ammunition goes through the concentricity tool.