How to Measure Bullet Seating Depth
The question of how long to seat a bullet in relation to the barrel’s rifling is an important one. Seating depth can have a lot of influence on how well your handloads perform. Knowing how to measure for it is an essential skill.
Luckily, it’s not that difficult. With a few basic measuring tools and some practice, we can accurately and consistently measure how far it is from the case’s head to the barrel’s rifling. We use that measurement to find the ideal seating depth for our rifle during load development.
In the video below, I’ll explain and demonstrate my process for measuring maximum Cartridge Base to Ogive Length (CBOL) and throat wear. I’ll also go over some considerations such as magazine length and where I like to start for bullet seating depth when I’m developing a load.
I’ll recap and highlight the important stuff after the video, and provide links to the tools I use for measuring seating depth. You can view the video right here, or click on the YouTube logo to watch it on our channel. Feel free to check out our other videos and subscribe to the channel while you’re there.
The two measurements discussed in the video are Cartridge Overall Length (COAL) and Cartridge Base to Ogive (CBTO). Both need to be considered when seating bullets.
COAL is measured from case head to bullet tip. CBTO is the measurement from the case head to where the bullet contacts the barrel’s rifling. The action and magazine length determine the maximum COAL. The Savage LRP shown in the video was engineered to allow the bullet to contact the rifling before running out of magazine length. Not all rifles work that way, so it’s important to know both measurements.
It should also be pointed out that reloading manuals base load data on SAMMI COAL specs. That’s an important consideration when basing your powder charges on that data. In any given caliber, the weight of the bullet will determine its length. The SAAMI COAL doesn’t differentiate between light and heavy bullets, it just lists one length.
Bullets used for long range shooting are typically on the heavy side and have long nose profiles. If you load them to SAAMI COAL, the bullet will be seated quite a bit deeper in the case than a lighter bullet. That bullet is now taking up case volume, limiting the amount of powder that can be used. It’s also likely to be jumping quite a distance to the rifling.
If the CBTO and magazine length will allow it, seating the bullets shallower will allow more powder to be used. This creates a lot of opportunities, and should explain why load data from experienced handloaders quite often doesn’t match “book data”. Before dumping more powder into the case, please realize there is more to a safe load than bullet seating depth. Proceed slowly, use a chronograph, and learn how to read pressure signs.
A question that came up soon after the video was published had to do with technique when checking maximum CBTO. Specifically, how much pressure to apply to the bullet when contacting the rifling.
As I mentioned in the video, this measurement is a relative value. It’s meant to be used as a comparative number to track throat wear and set optimum seating depth. The shape of the bullet’s nose and how it contacts the lands of the rifling will affect the “feel”. The more you use the overall length gauge, the more comfortable you’ll get with the readings.
I push lightly on the rod until it stops, then hold it there while tightening the screw. The bullet will usually stick in the rifling, more so when measuring with VLDs. Inverting the rifle and lightly tapping the butt of the stock on the bench should dislodge it. I usually take two or three readings and write them down. If they match, I know my technique is repeatable.
The most important things are to make sure the bullet moves freely in the neck and use the same bullet insert for every reading. Practice taking measurements until you’re comfortable with your method.
For detailed information on the comparator and inserts, click HERE. I’ve been using the same Mitutoyo (CD-6” CSX) electronic caliper for a long time now. I highly recommend it. Just about everybody stocks the Hornady OAL Gauge and modified cases. If you want to make your own modified cases, the tap size is 5/16”x 36 tpi.
Knowing how to consistently and accurately measure bullet seating depth is a critical step in the reloading process. The methods I used in the video and explained in this article are the same ones I use for every rifle I handload for. Give them a try and see if they work for you!