Imagine a centerline running straight through your bullet. Start at the primer and run this line right through the tip of the bullet.

In theory,if your case is straight and your bullet is set straight in the case, then it is what is known as "concentric", meaning that there is no deviation from the centerline of that bullet. This will be the most accurate ammo and this is what everyone strives for, but in practical application, it is not as easy to achieve as it seems.

In reality, there is always a bit of deviation from the centerline and this is what is known as "runout".

Runout can occur from several things. Proabably the most common that comes from reloading is that the dies are not set true to the ram of the press. Most dies are threaded at 7/8 -14 TPI. Theseare threaded into the top of the press. The problem is thatthere must be a certain amount of clearance for the parts to be threaded together.There is enough clearance in the threads thatif the die is not set correctly, thedies will actually lean to one side and not besquare to the ram.If so, the bullet will be forced by the die a bit crooked into the case and it will cause "runout".

The proper way to eliminate that runout is the set a case into the shellholder and bring it up into the die which has not been tightened. You then tighten the locknut on the die and this squares the die to the ram and allows the bullet to go into the case as straight as it possilby can.

For some reason, many reloaders either bypass this step, or dont know to do it.

Another type of runout which can affect cases is the runout of the chamber to the rifling. If the chamber in either a rifle or pistol barrel is not bored concentric to the rifling, the fired case will assume the dimensions of the chamber. Unless that case enters the chamber exactly the same way, it may not fit. In the case of relaoding, passing it through a sizing die will usally take care of it, but it alos works the brass more and that brass wont last near as long when being fired through that chamber as it would if were being fired through a chamber that was perfectly centered up, or "concentric".

A way to measure runout on yout bullets it with a case measuring tool. This has a dial indicator set up on a post and it measures the case and the bullet that is set on two V-blocks. You put a bitof tension on the dial indicator and set the face to zero, and then you gently turn the case. If your indicator needle dosent move, then its good. You then positon the indicator to set on the bullet and reapeat the steps. You may see the indicator needle move a few thousanths of an inch. If there is any deviation it is runout. It may help to take a marker and mark the high spot and then watch the relationship of it to the bullet. If there is more than a few thousands of runout, you need to start looking at your reloading proccess and make some corrections.