Bullet Concentricity & Related Issues | Top Rated Supplier of Firearm Reloading Equipment, Supplies, and Tools - Sinclair Intl

Bullet Concentricity & Related Issues

The purpose of loading your own ammo is to minimize all the variables that can affect accuracy and can be controlled with proper and conscientious handloading. Concentricity and bullet run out are important when you're loading for accuracy. Ideally, it's important to strive to make each round the same as the one before it and the one after it. It's a simple issue of uniformity.

The reason shooters work with tools and gauges to measure and control concentricity is simple: to make sure the bullet starts down the bore consistently in line with the bore. If the case isn't properly concentric and the bullet isn't properly aligned down the center of the bore, the bullet will enter the rifling inconsistently. While the bore might force the bullet to align itself with the bore (but normally it doesn't), the bullet may be damaged or overstressed in the process - if it even it corrects itself in transit. These are issues we strive to remedy by handloading, to maintain the best standard possible for accurate ammunition.

The term "concentricity" is derived from "concentric circle". In simple terms it's the issue of having the outside of the cartridge in a concentric circle around the center. That goes from case head and center of the flash hole, to the tip of the bullet.

Factors Affecting Concentricity

The point of using this term is to identify a series of issues that affect accurate ammunition. Ideally this would work best with a straight walled case; but since most rifle cartridge cases are tapered, it equates to the smallest cross section that can be measured point by point to verify the concentric circle around the center. For the examples below, I'm working with .308 Winchester ammo.


Figure 1: The cartridge.


Figure 2: Centerline axis of the case, extending from flash hole to case mouth.

The case walls have to be in perfect alignment with the center, or axis, of that case, even if it's measured at a thousandth of an inch per segment (in a tapered case).


Figure 3: Case body in alignment with its axis, or centerline, even in a tapered case.

The case neck must also be in alignment with its axis. By not doing so you can have erratic bullet entry into the bore. The case neck wall itself should be as uniform as possible in alignment and in thickness (see the M80 7.62x51mm NATO cartridge in Figure 5) and brass can change its alignment and shape. It's why we expand the case neck or while some folks ream the inside of the neck and then turn the outside for consistent thickness, which affects the tension on the bullet when seated.


Figure 4: Neck in alignment with center of the case axis.


Figure 5: Variations in case neck wall thickness, especially on some military brass, can cause an offset of the bullet in its alignment. This is an M80 ball round. Note the distinct difference of the neck walls.


Having a ball micrometer on hand helps, especially with military brass like 7.62 in a semi auto rifle, where there are limits as to how thin you want the neck walls to be. In the case of 7.62 ball brass you want to keep the wall to .0145".



Figure 6: A ball micrometer like this one from RCBS (#100-010-268) is a big help in measuring case neck thickness.

Turning the outside of the neck wall is important with .308 military cases regardless of whether you expand or ream the neck walls. There are several outside neck turning tools from Forster, Hornady, Sinclair, and others. I've been using classic Forster case trimming (#100-203-301) and neck turning (#749-012-890) tools for 40 years.

Bullet Run Out

The cartridge, after being loaded, still needs to be in alignment with the center of the case axis. Figure 7 shows a bad example of this, a round of M80 ball. A "tipped" bullet is measured for what's known as bullet "run out".


Figure 7: An M80 round with the bullet tilted and not aligned with the axis. This will be a flyer!

Run out can be affected by several things: (1) improperly indexing your case while sizing, which includes not using the proper shell holder, especially while using a normal expander ball on the sizing die (it also can stretch the brass). (2) The head of a turret press can flex and (3) improper or sloppy bullet seating. This is also relevant when it comes to using a progressive press when trying to load accuracy ammo.

Mid Thompkins came up with a simple solution for better bullet seating years ago. Seat your bullet half way into the case, back off the seater die and rotate the case 180 degrees before you finish seating the bullet. It cuts down on run out problems, especially with military brass. You also want to gently ream the inside of the neck mouth to keep from having any brass mar the surface of the bullet jacket and make proper seating easier. A tipped bullet often means a flyer.


Figure 8: Proper alignment from the center of the case head to the tip of the bullet.

Because of the issues involved, I use the Forster Co-Ax press (#749-008-316) for several reasons. Its built-in shell holder can move side to side and the dies can move forward and backward - this means the press self-indexes. This also means that changing the dies is as fast as taking them out and putting in new ones, once they're set up. This includes changing to different calibers (I do change from .308 to .30-06 regularly). The lines on the Co-Ax press shown in Figure 9 show you the movement of the die and shell holder.


Figure 9: The Forster Co-Ax press is self-indexing.

Total Indicated Runout

Bullet run out means simply means the way the bullet “runs out” into the rifling. The ideal desired cartridge concentricity is zero Total Indicated Runout (TIR), measured with a concentricity tool with the dial indicator on the bullet about 1/16th of an inch above the case mouth. If you want to get really technical, do the case neck when using military brass, so you can check the concentricity and any variation in case neck walls. Do the measurement on the case neck slightly below the case mouth. Factory loaded match ammunition usually exhibits no more than .0015" TIR, which is deemed excellent.

If you load for accuracy, especially for distance shooting; you want no more than .002" to .003". Ammo with a TIR of .005" is used for shorter ranges. If it goes beyond .005", either use it for practice ammo or very short ranges because accuracy will fall off quickly with that kind of TIR. Even M118 7.62 NATO ammo had problems with TIR depending on the lot, from early M118 to M118LR.

For a case and cartridge inspector, I started with the classic Forster Case and Cartridge Inspector (#749-007-261), which a lot of people still use. The Sinclair Concentricity Gauge (#749-007-305) is an exceptional tool for this task, too.


Figure 10: The exceptional Sinclair concentricity gauge - very handy for measuring TIR with great precision.

At the moment, I happen to be using the Hornady Lock-n-Load Ammo Concentricity Tool (#749-007-534), which also does a great job.



Figure 11: Hornady's ammo concentricity gauge, also a good choice.


Once you have the round in the concentricity tool, set the indicators for quick reference, to check the variations indicator movement to see the TIR. The indicators in Figure 12 are set for .003", but it's simple to just watch the differences in the indicator movement to see variations that give you the true TIR.


Figure 12: Checking the indicator movement.

It's not perfect, and it won't really correct bullet run out with .7.62x51 military cases because the case walls used in semi auto rifles are thicker than those used for benchrest or F class rifles, but it works nicely and is easy to use.

A simple example of military M118 match ammo run out gives you an idea of the quality of the early production versus late production where the accuracy was fairly bad.

Bob Kohl