There is a vast lack of uniformity in comparators and measuring devices used to determine Cartridge Base to Bullet Ogive (CBTO). This is a critical point to understand. To measure from the base of the cartridge to where the bearing surface ends on the bullet you must use a gauge that will attach to your calipers and which also goes over the nose of the bullet to touch the point where the bearing surface transitions into the nose curve. We already sorted out that bullets can and will vary in this area (at least from type to type if not lot to lot). This makes it impossible for gauge manufacturers to use one given diameter and shape in there gauges. So there is no standard shape and diameter for gauges. Said differently, gauges can and will vary in both inside diameter and the shape where the gauge contacts the bullet.
There is another reason why these gauges are not standardized. Since bullet nose shapes and diameters will vary, gauge manufacturers know that gauge standardization is impossible. Since this is true the end result is that this measurement becomes a comparison used by one shooter rather than a consistent dimension used by many shooters. Given this fact, they are free to open their tolerances up from gauge to gauge. Anyone who understands tooling knows that it is much cheaper to make a tool with a larger tolerance window.
Some of you might be saying, “Hold on a second, if the gauge can vary then how can anyone use CBTO successfully?” The answers is because since this dimension cannot (or is not) standardized the specific CBTO dimension used by one shooter is critical but this dimension is likely not to match the specific dimensions of a cartridge shot by another shooter. “Huh?” you say? Let me explain.
If you have one gauge and you are shooting one lot of bullets, you have the ability to measure and adjust CBTO to get the most performance out of your rifle. All of the dimensions using your gauge and bullets are meaningful to your rifle. Testing to find the best CBTO is a key part of getting the most precision from your rifle and handloads.
For example, suppose that your CBTO using a 308 Winchester is 2.110”. You take this to the range and it shoots like a house a fire (shoots great). If you call your buddy up and tell him that he should try a CBTO of 2.110” in his rifle he will be grateful until he goes to the range. When your buddy who has a different rifle/chamber, is using a different bullet (type or lot) and different gauge sets up his cartridge to have a CBBS of 2.110” he will expect the same level of performance. But his rifle doesn’t shoot well at this CBBS dimension. You both are puzzled until you try something.
You take your gauge and your bullets over to his house to find out what he has done wrong. The first thing you do is you measure the CBTO of his ammo. This is when you find the first problem. His CBBS is 2.074”. Just as you start to give him a hard time for getting it wrong he pulls out his gauge and measures his ammo again. When he does it with his gauge he gets 2.110”. In this scenario, the difference is due to the fact that your gauges are not the same.
Trying to sort it out further, you decide to load some of your bullets into his cases with his seating die set up exactly the same. Then you should be able to get the same measurement, right? You load one round and take a measurement. With your bullet at his seater die setting your CBBS is at 2.093”. When he measures this cartridge with his gauge he gets 2.057”. What the heck? Now you both are all over the place. This second attempt to get things sorted out is thwarted by the fact that the nose shape of your bullets is different than the nose shape of his bullets. You both decide that this is a waste of time since the variation is so much. How can something that varies so much be important to performance?
This simple answer is that you have to apply it correctly and to your rifle using your own gauge and your own bullets. The first step is to establish the distance from the bolt face to the rifling. How is this done? There are a two most common ways and neither is without difficulties. The most consistent and accurate way is to load a cartridge purposefully long using medium to light neck tension. When you chamber the round and close the bolt the bullet gets pushed into the case. If you slowly open the bolt and remove the cartridge it should be a representation of the distance from your bolt face to where the bearing surface of the bullet engages the rifling. You need to do this several times because with medium to light neck tension the bullet may pull back out of the neck if it is wedged too tightly into the lead angle of the rifling. If you do this several times and come up with the same dimension (within .001) you can call it good.
There are a few things you need to be aware of when using this method. It is important that you use exactly the same bullet each time. Not the same type of bullet or same lot but the exact same bullet. If the neck tension is light enough you should not change the shape when you pull it for another measurement. You also need to measure the COAL to make sure the bullet moved in the first place. You may seat it long thinking that your throat couldn’t be longer than this COAL but find out that when you do this check the bullet doesn’t move at all. This indicates that either the bullet pulled back out when you opened the bolt or the bullet was not out far enough to touch the rifling. The more you do this check the better you will get at doing it well.
The other common way to get this dimension is to use the Stoney Point (or Hornady) Overall Length Gauge. This is a device that allows you to push a case into a chamber that holds a bullet in the neck loosely. After the case is inside the chamber you push the bullet forward with a rod until it stops at the rifling.