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.
Overall length gauge with modified case.
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!
must be a bad winter in Idaho lol
It’s certainly dragging on! It was snowing again this morning. I’m ready for spring.
Thanks for the excellent video description! I’ve been using the “lite-press-fit of the bullet” technique but I might have to get me one of those gauges you use. I can see where that would give a more sensitive measure of when the bullet is just touching the rifling.
Sam,
When making your own modified case, do you set back the shoulder or leave it as it came out of the rifle? I am building up a savage Lrp in 260 and am waiting on the tap to make a modified case. Looking forward to your review of your savage lrp.
Keep up the good work!
I run the case through a body die or FL bushing die with the bushing removed. It makes it much easier to ensure the shoulder is contacting the chamber. I’ve much much better consistency with this method.
Sam you stated for seating depth your starting point is .0010 off the lands. Next do you step closer to the lands? If you step towards the lands is your stoping point @ the lands? I appreciate your knowledge and the ability to clearly teach others. Thank you, Mark
I start at .010″ off. I always go further out before trying at the lands. I’ve only had one barrel that I can recall, that liked to be close. I never jam into the lands.
Hello Sam,
I’m new to this forum,so if this question is in the wrong place forgive me.What criteria do you use for case selection,and do you sort before or after any work is done to the case,such as trimming ect?Thanks,Scott
Most of my process is in “New Brass Prep”. 1-2% is a generally accepted tolerance when weighing cases. I would definitely trim them before weighing.
Hi Sam great videos I am learning a lot from them. Just looked at your last one about the primer pockets. RCBS Lyman and Hornady make a tool that is called. Pocket uniforming cuter. Keep up the good work thank you for the videos
Hi Sam keep up the good videos. I just got into long range shooting I had been reloading for several years for planking and handguns. This is new to me so I am trying my best. The tool you were showing on your video is called a pocket uniforming cutting tool. They are made by Hornady Lyman and RCBS not very expensive. Keep up the good work on the videos and I am learning a lot. Thank you Sam hope this is helpful to you or other viewers.
If you are trying to make a Berger VLD shoot what size steps do you decrease COAL?
Thanks — Todd
I usually move in .010″ increments with any bullet I’m trying. If I don’t find a spot within a .100″ bracket where they’ll shoot, I’ll probably abandon the bullet. That’s assuming I’ve exhausted powder and case neck tension options.
Sam, great job on all your vids. I’m in the process of building my first LRP Remington in 7mm-08 and I find that the information you provided is invaluable. You have saved me so much time and money . Can’t thank you enough!
Sam,
Thanks for all the great information you are putting out there. After watching your video on bullet seating I am running into mixed results using the COAL gauge. I have taken dozens of reading trying to figure out my barrel on a SAVAGE 110 FPC .338 LAPULA MAG. I keep coming up with different readings. ( 2.907,2.905,2.903,2.910) 2.910 seems to be the most common reading. Am I safe in assuming that 2.910 is a good starting point ?
I usually look for a few of the same measurements, but all that matters is consistency. An average would also work, so you could use 2.906 if you want to do it that way. It took me a few tries to get comfortable with the whole process, but now I have no problems with getting the same exact value over several measurements.
Sam,
Thanks for taking the time to do this. I appreciate your sharing of knowledge.
I was CBTO measuring some .223 for the first time on 2 different rifles and I believe my process is fine. The thing is, if I measure the COAL afterward, it is a number shorter than SAAMI max COAL. Does this make any sense? As an example, I was making a 55gr SP 2.200″ COAL according to my manual and after gauging and checking CBTO, I cross ref’d COAL to be 2.163″. So, maybe my process needs improvement. Or maybe with that particular bullet, it happens to be shorter. It seems like if I push more firmly on the gauge, I might pop the bullet right out. I guess it would be highly unlikely that I’m doing this correctly if this is the case. If I push harder, the bullet does not come out easily. I have had to put a rod down the wrong way.
If you know anything about this, I would appreciate any kind of help.
Respectfully,
SAAMI specs are drawn up when a cartridge is designed, and given a standard COAL with a particular bullet. That spec is used everywhere; for rifle manufacturers, magazine dimensions, ammo producers, and loading manuals. As you can imagine, there are a lot of different bullet dimensions across manufacturers, especially in the nose profile. Not only that, if you take 10 bullets from any given box and measure the OAL of the bullet, you’ll likely end up with several different values. Most of the variation will be in the nose length, from the shoulder of the ogive to the tip. That’s why measuring for a consistent CBTO works better for accuracy than trying to even out COAL. If you look at the loading manuals, the same COAL is listed for that cartridge for every bullet used for load data.
It’s a great question, and one I’ve answered a few times this month. I’ll use it in one of my future “Straight Talk” videos. The short answer is don’t worry about COAL, as long as the bullet’s bearing surface is above the neck and the tip doesn’t interfere with magazine fit. Focus on CBTO for accuracy and consistency.
Good to hear.
I really was paying attention to details but it never occurred to me that I‘d end up going shorter. I guess that means the rounds I started out with would’ve been pressed up tight. I am actually really excited to start experimenting with this.
Again, I thank you.
Adrian.
When you find the ogive measurement for your rifle, is this the measurement you use for all grains/brands of bullets? Or do you have to find the ogive measurement for each grain/ brand?
I generally check the CBTO with the bullet I want to try. Some bullets contact the rifling the same way, others are a little ways apart. I have shot a bunch of different 140s through my 260 Remington at exactly the same CBTO. They all shot well.
Would it be better to send once fired cases to Hornady for modification instead of buying their new modified cases?
That’s completely up to you: https://www.youtube.com/watch?v=I-ADGQJFn70&t=1881s
I bought the Hornady O.A.L gauge and the bullet comparation. I shoot a round in the gun, then bump the shoulder back .002″. Then I drilled & tap the case for the O.A.L gauge. Then I check the CBTO, should this be the same for all bullets in this gun? I have 3 different bullets and I get 3 different CBTO one for each bullet. I thought it would or should be the same. Can you clarify for me.
Enjoy the good information.
Not necessarily. I would use the bullet you plan to shoot for your max CBTO and initial seating depth. I usually find that even though different bullets vary in max CBTO measurements, they shoot well at the same seating depth. For instance, if a 142 SMK comes in at 2.037, and a 140 ELD measures 2.030, they’ll likely both shoot well at 2.020″.
Love your content and videos. Trying to figure out how to Setup/compare my CBTO with a Hornady case with a once fired case. Would it make sense to measure the shoulder of a fire formed case to the Hornady case and add that difference minus shoulder bump to my CBTO measurement I get from the Hornady case? I don’t have the ability to tap a fired case and thought this would help quantify the difference from a fired case vs the Hornady case. Seems you’d over seat the bullet in the fired case…or maybe I’m all wrong. Thanks!
I under stand how to measure bullets but cant understand why. weight sorting I understand. when the seating die makes contact with the bullet, the seating depth is the same on all…yes??? I’m using a Sinclair Bullet Comparator separating them by .0005 the dies are Competition dies by Redding thank you
Bill – the seating depth is not necessarily the same. I use the Sinclair bullet Comparitor also. It touches the bullet close to where the rifling would contact the bullet. But when you seat a bullet, the bullet is being pushed down with the Seating Stem. The seating stem contacts the bullet at a different point than the Sinclair bullet comparitor. The difference in bullet contact with the seating stem vs the bullet comparitor could cause a difference in seating depth.
Pound for pound you got to be on the top three list-if not number 1- for best reloading website.
Appreciate you share it with the public in a practical manner.
Quick question in regards to determining seating depth from new case to once fired. I am loading 308 and I will be FL sizing my brass first go around because the brass was used in two diff. guns from facetory gold medal match and got mixed together. I will use the Hornady COAL tool to find my COAL. From there I will have to add/subtract headspace of the Hornady case vs my FL sized case to get my CBTO measurement. I determine I want them .020 off the lands on both firings of my FL sized brass and then my FL bump size (.002) brass. I have been playing with numbers on paper but it seems I just keep seating the bullet longer with longer brass. How do I ensure my bullet is seated the same from my FL size brass on my first reload to my bump sized brass on the second loading? Is there a math formula that I can use to get the lengths I need?
Any idea why Hornady say to NOT resize the brass when sending them a fire formed case for modification? It doesn’t appear on their webpage that they do that prior to sending back.
Probably because people would run them through a FL sizing die and size the neck down, making it worthless as a modified case.
Sam,
New to your website and reloading. Thanks for making these informative videos and posts, as it really fleshes out the nitty gritty that isn’t covered in reloading manuals.
A question I do have is in measuring the bullet seating depth on a reloaded cartridge.
From what I gather from watching I would make these measurements from the video, and then when reloading cartridges, adjust my seating die to a shallow depth and seat the bullet, then measure the CBTO on the reloaded cartridge. From there I assume I would adjust the die incrementally taking measurements until the die has been adjusted to the desired seating depth using the bullet comparator to measure the CBTO on my reloaded cartridge as compared to the OAL gauge measurement to the lands minus whatever my target depth was. I’m pretty sure this is the way to do it based on what you describe.
Sam, How would this process differ for a belted magnum? I’m going to start reloading .300WM. Since the case is seating off the belt and not the shoulder. Or does that change once brass is fire formed?
First firing is supported by the belt. All subsequent firings are determined by headspace dimensions and shoulder setback. If headspaced correctly, it shouldn’t vary by more than ~ .002″. Not much benefit to a custom modified case when it comes to belted cases, other than convenience.