Convert COAL to CBTO

[hollywood1981]

Is there a math conversion to convert for an example, if im reloading a 308 with a COAL of 2.800 with an 180 grain bullet. What would the measurement be from the ogive when using a comparator? I'm asking because I'm reloading 308 to mag length of 2.800 COAL. When measuring my first round seated, I adjusted my seating die to this setting. Seat the next bullet and the COAL was shorter. I know the tips of the bullets are not uniformed, and I will have a variance in COAL vs CBTO. What should I do in this instance? I just want to set my die, I do not want to move it around everytime I have to seat another round. Any insight will help thanks.

 

[Dthomas3523]

Small variation won’t matter much.

And you’ll have to use comparator to get cbto. If you’re going to use 2.8 OAL as your reference, take a few and seat them to 2.8 OAL, then measure cbto on them and take the average or most common cbto as your number set your seating die.

Ideally, unless you’re limited to mag length you’ll just use cbto for everything. OAL is typically a measurement only used to share information as all comparators are different and cbto from person to person is worthless.

 

[avidflyer]

couple things, seater shouldnt touch the tip, best works touching the ogive. second you need to figure out if its touching the tip and what going on with your consistency. is the seater adjustment portion locked down. how much variation is in the bullets...measure base to ogive for that number. all the same lot? its a start

 

[Shootin Stuff]

As long as they will all fit, feed and eject from your mags then COAL isn’t important as they all have small tip variations.

CBTO is what you are after and you need a comparator of some sort to do this.

If you don’t have a comparator then even a socket that fits on the ogive will work to give you a measurement.

As long as you have a dummy round seated to a known depth from the lands then even the length of comparator doesn’t matter if you re zero your calipers to the dummy.

 

[hollywood1981]

I have an OAL comparator from honady, that I use for my other calibers to measure my CBTO. Also I am limited to mag length in this case. The loads are for a gas AR10 rifle, not a bolt gun with magizine.

 

[Shootin Stuff]

Then seat to a coal that will work in your mags and forget about the tip variances, CBTO is the only one that matters now.

 

[918v]

It’s hard to explain but here goes:

What you did is adjust your die to seat one bullet to 2.800”. If you’re using good bullets, all of them will have consistent cbto measurements. They may not be the same at the tip, but they will be the same at the ogjive. Therefore that same die setting will produce consistent cbto numbers. To make cbto changes, use the die adjustments and forget about the tip as long as you have room in the mag.

 

[hollywood1981]

It’s hard to explain but here goes:

What you did is adjust your die to seat one bullet to 2.800”. If you’re using good bullets, all of them will have consistent cbto measurements. They may not be the same at the tip, but they will be the same at the ogjive. Therefore that same die setting will produce consistent cbto numbers. To make cbto changes, use the die adjustments and forget about the tip as long as you have room in the mag.

Thanks for the help! I was just over thinking the whole thing I guess. Appreciate it

 

[straightshooter1]

Is there a math conversion to convert for an example, if im reloading a 308 with a COAL of 2.800 with an 180 grain bullet. What would the measurement be from the ogive when using a comparator?

Various bullets will have various designs where the relationship of their ogives to their COALS are just as diverse. So, the answer is there is no universal math conversion that you can use to give you the BTO measurement. The best way to find the CBTO is with a comparator like the Hornady's that many reloaders use.

I'm asking because I'm reloading 308 to mag length of 2.800 COAL. When measuring my first round seated, I adjusted my seating die to this setting. Seat the next bullet and the COAL was shorter. I know the tips of the bullets are not uniformed, and I will have a variance in COAL vs CBTO. What should I do in this instance? I just want to set my die, I do not want to move it around everytime I have to seat another round. Any insight will help thanks.

When you get a box of bullets, it's a good idea to measure a good sample of their BTO's to get an idea how consistent they might be. Quality bullets (like Berger's) will have very small variances, something like .002 - .004. With other manufacturers there BTO's can have variances well over .030. If there's a large variance, then it's a good idea to short them so that you can get more consistent seating depths.

Another issue that effects seating depth: If there's a lot of resistance or a lot of variance in resistance when seating bullets, you're going to get quite a variance in seating depth. There are several things than can effects this resistance and reloaders address the details of their brass prep so as to get smooth consistent seating pressure to get consistent seating depth.

Note that seating dies do not have seating pins that can reach down to the ogive. So, they make contact with the bullet somewhere above the ogive as there's no room at the bearing surface ogive junction. If there's a large difference in the BTO's in your lot of bullets, then sorting them by the BTO measurments you get with a comparator it really your best option.

You shouldn't have to keep adjusting your seating die to get a consistent seating depth, which is measured with a comparator to a CBTO (Case Base To Ogive).

Hmmm??? You said your .308 mag length is 2.800, which doesn't quite seem right to me. 2.800 is SAAMI spec for a .308 and all mags I've seen are longer than that. And it's common for .308 reloaders to load longer than 2.800.

 

[hollywood1981]

Various bullets will have various designs where the relationship of their ogives to their COALS are just as diverse. So, the answer is there is no universal math conversion that you can use to give you the BTO measurement. The best way to find the CBTO is with a comparator like the Hornady's that many reloaders use.




When you get a box of bullets, it's a good idea to measure a good sample of their BTO's to get an idea how consistent they might be. Quality bullets (like Berger's) will have very small variances, something like .002 - .004. With other manufacturers there BTO's can have variances well over .030. If there's a large variance, then it's a good idea to short them so that you can get more consistent seating depths.

Another issue that effects seating depth: If there's a lot of resistance or a lot of variance in resistance when seating bullets, you're going to get quite a variance in seating depth. There are several things than can effects this resistance and reloaders address the details of their brass prep so as to get smooth consistent seating pressure to get consistent seating depth.

Note that seating dies do not have seating pins that can reach down to the ogive. So, they make contact with the bullet somewhere above the ogive as there's no room at the bearing surface ogive junction. If there's a large difference in the BTO's in your lot of bullets, then sorting them by the BTO measurments you get with a comparator it really your best option.

You shouldn't have to keep adjusting your seating die to get a consistent seating depth, which is measured with a comparator to a CBTO (Case Base To Ogive).

Hmmm??? You said your .308 mag length is 2.800, which doesn't quite seem right to me. 2.800 is SAAMI spec for a .308 and all mags I've seen are longer than that. And it's common for .308 reloaders to load longer than 2.800.

I measured my p mags with my digital calipers, and my measurement i got was 2.810 max.

 

[918v]

If your seater die doesn’t have a micrometer adjustment then you can measure the length of the die using your dial caliper. You can adjust the seating depth / cbto changes pretty accurately that way.

 

[Pink_Mist]

I used to use the modified case tool but now I remove the firing pin and ejector pin from my bolt. I seat a bullet long in a fired, neck sized only case and gradually increase seating depth until the bolt handle will fall half way closed by gravity alone. This has produced more consistent results than using the tool. My point - Is this method is possible in a gas gun too.

 

[Shootin Stuff]

I used to use the modified case tool but now I remove the firing pin and ejector pin from my bolt. I seat a bullet long in a fired, neck sized only case and gradually increase seating depth until the bolt handle will fall half way closed by gravity alone. This has produced more consistent results than using the tool. My point - Is this method is possible in a gas gun too.

Yes, you can also slit a neck with a thin cutoff wheel and gently close the bolt. The neck tension will be loose enough that it will push the bullet back once it hits the lands, but tight enough that it will not shift when you carefully remove and measure.

Edit: it is also quite possible you may not reach the lands even with the bullet barely seated. In which case you know you can load that projectile to whatever length you want and still not be jammed.

 

[Pink_Mist]

Yes, you can also slit a neck with a thin cutoff wheel and gently close the bolt. The neck tension will be loose enough that it will push the bullet back once it hits the lands, but tight enough that it will not shift when you carefully remove and measure.

Right but strip the bolt, I don't like to fight the ejector spring

 

[Shootin Stuff]

Right but strip the bolt, I don't like to fight the ejector spring

Yep, strip the bolt.

 

[FarmBoy87]

The only time I ever measure coal is when it becomes an issue...always use cbto unless forced to use coal due to it being a constraint.

 

[FarmBoy87]

Also about making a modified case...I have formed similar cases into modified cases as long as they are very close...use a headspace comparator to verify and then use an oversize expander and that will allow the bullet room to move.

 

[straightshooter1]

I measured my p mags with my digital calipers, and my measurement i got was 2.810 max.

Hmmm??? Interesting.

For my 7.62 X 51 PMAG 10's, I can get a cartridges in them with COAL's of 2.835 at the max. And, it seems some will go even longer to 2.86 :

Magpul PMAG 10 7.62AC AICS Short Action Magazine, 7.62x51mm NATO, 10 Rounds

Sportsman's Guide carries top-quality discount Outdoor and Hunting Gear, Guns, Ammo, Fishing Supplies and more - all at great low prices!

www.sportsmansguide.com

 

[spife7980]

Hmmm??? Interesting.

For my 7.62 X 51 PMAG 10's, I can get a cartridges in them with COAL's of 2.835 at the max. And, it seems some will go even longer to 2.86 :

Magpul PMAG 10 7.62AC AICS Short Action Magazine, 7.62x51mm NATO, 10 Rounds

Sportsman's Guide carries top-quality discount Outdoor and Hunting Gear, Guns, Ammo, Fishing Supplies and more - all at great low prices!

www.sportsmansguide.com

Gas guns don’t use aics pattern mags.

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Looks to be 2.830 max oal according to manufacturers specs. Load some to 2.820 and see that they feed and then what they measure to the ogive with a comparator and you now have your base line practical max seating length.