CBTO vs. SAAMI COAL

OK, so I'm a newbie and just did my first batch of rounds following Hornady's directions since I was loading SST's. But I've just figured out the importance of seated bullet depth--rather more specifically getting the bullet's base of ogive as close as possible to the lands in the barrel's throat. I just followed the COAL that hornady has in the book--but when preparing to load Nosler bullets I came across a totally different COAL for the same caliber (which I'm sure is due in part to the different bullet) but my initial reaction was "waoh, how could they be that different?" That's when I found the CBTO info and realized that this was a very important issue in getting my loads accurate.

So, my question is --how many of you go by CBTO rather than SAAMI max, and what do you find is the best way to determine CBTO for your rifle? I'm also aware of the importance of magazine clearance.

Thanks, : )

First you need to determine the jamb length for the bullet you want to use in your rifle. Easiest way is to place the bullet in a sized, uprimed case (obviously without powder); seat the bullet a little long. Bolt into the chamber to "jamb" onto the lands; then eject but care must be taken to assure the lands don't grab the bullet and cause an error. I oil the bullet slightly and gently use a cleaning rod down the muzzel when ejecting to facilitate this process. While you can use the overall length, a comparator based on ogive measurement is better. Once you know the jamb length then you can try different seating depths.

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Originally Posted by CharlieNC

First you need to determine the jamb length for the bullet you want to use in your rifle. Easiest way is to place the bullet in a sized, uprimed case (obviously without powder); seat the bullet a little long. Bolt into the chamber to "jamb" onto the lands; then eject but care must be taken to assure the lands don't grab the bullet and cause an error. I oil the bullet slightly and gently use a cleaning rod down the muzzel when ejecting to facilitate this process. While you can use the overall length, a comparator based on ogive measurement is better. Once you know the jamb length then you can try different seating depths.

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Thanks a bunch ; )

My next question related to this is if you are doing a powder fill that doesn't approach full, isn't the vacant space in the case with a bullet seated long going to reduce muzzle velocity significantly?

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Originally Posted by thermaler

Thanks a bunch ; )

My next question related to this is if you are doing a powder fill that doesn't approach full, isn't the vacant space in the case with a bullet seated long going to reduce muzzle velocity significantly?

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There will be a difference. That is why there are so many different powders and burn rates. Using fast pistol powder you need less powder, but your case would be very empty while still achieving high pressure. Cross the magic line where there is to much air space with the powder and you have an explosion in the case rather then a controlled burn. To slow a powder for your case size and awesome fireballs with no speed. 85% is what many think the case should be filled to for a generic magic number.

So back to your specific question. You will have to play with the powder charge a bit to adj between bullets of the same weight with different shapes to maintain velosity. The long sleek boat tail bullets are going to be seated deeper then a flat base bullet of the same weight. Another rule of thumb. Bullets are to be seated equile to there diamiter.

Hornaday makes tools for finding the proper seating depth for different bullets with out having to jamb them into the gun, But they cost money. Some will put a slight pinch in a fired case to make the bullet easier to slide and avoid to much pressure causing the bullets to stick in the barrel.

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Originally Posted by jibben

There will be a difference. That is why there are so many different powders and burn rates. Using fast pistol powder you need less powder, but your case would be very empty while still achieving high pressure. Cross the magic line where there is to much air space with the powder and you have an explosion in the case rather then a controlled burn. To slow a powder for your case size and awesome fireballs with no speed. 85% is what many think the case should be filled to for a generic magic number.


So back to your specific question. You will have to play with the powder charge a bit to adj between bullets of the same weight with different shapes to maintain velosity. The long sleek boat tail bullets are going to be seated deeper then a flat base bullet of the same weight. Another rule of thumb. Bullets are to be seated equile to there diamiter.

Hornaday makes tools for finding the proper seating depth for different bullets with out having to jamb them into the gun, But they cost money. Some will put a slight pinch in a fired case to make the bullet easier to slide and avoid to much pressure causing the bullets to stick in the barrel.

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Thanks for the rely--I love learning about this stuff. I'm going to go with the "jamb" technique--I'm just a little apprehensive about leaving a bullet behind in the bore using the other technique. The Nosler book does a great job of describing this stuff. Once you get your measurement, do you basically just back off your die setting about .001 or so from CBTO before seating your bullets--assuming you don't want the ogive base to actually be engaged in the lands upon chambering?

The bullet seating die has an adjustment knob on the top so you can easily fine-tune the seating depth. A good nominal setting to try is .020" jump off the lands for many bullets; you don't want anything close like .001 because minor differences will mean the bullet may actually touch the lands sometimes, so that .001 has a major impact when seating that close. On the other hand many competitive shooters like .005-.010 into the lands (so the bullet "seats" itself) but good luck removing an un-fired round when field shooting (your bullet sticking in the bore scenerio).

The Nosler book has a great technique for "jambing": use a fired case, put a bullet in (I tapped the case mouth a few times on cement to get just barely some grip) color the bullet with a magic marker, insert into chamber and close bolt. Remove cartridge and pull bullet out and realign line where marker ink is scraped away. Measure, and repeat 3 times to average results. They also recommend an offset from lands in the range from .03 to .015--I was thinking the safe thing to do would be start with the .03 and work up to .015--does this make sense?

You will not see any difference in pressure sign unless you are already way over. If your working from the book you should be good. The seating depth will affect your accuracy, but with out a crony you will not see any other difference. There are 2 or 3 good accuracy nodes with powder changes alone. Find a slow or midrange charge to play with that gives good accuracy and then experiment with depth.

Thanks--I was wondering if there was a methodology for working up the seating depth--sounds like find the best powder/bullet combo first and then fine tune with seating depth depending upon velocity requirements (which is a big deal to me, since I do intend to hunt with my loads and don't want a deer running off with an inadequate impact).

The differences in terminal effects within the velocity range normal between a starting load and max load are going to be insignificant. Find an accurate load. Practice putting the shot where it needs to go. The terminal effects will be sufficient from there.

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Originally Posted by thermaler

OK, so I'm a newbie and just did my first batch of rounds following Hornady's directions since I was loading SST's. But I've just figured out the importance of seated bullet depth--rather more specifically getting the bullet's base of ogive as close as possible to the lands in the barrel's throat. I just followed the COAL that hornady has in the book--but when preparing to load Nosler bullets I came across a totally different COAL for the same caliber (which I'm sure is due in part to the different bullet) but my initial reaction was "waoh, how could they be that different?" That's when I found the CBTO info and realized that this was a very important issue in getting my loads accurate.

So, my question is --how many of you go by CBTO rather than SAAMI max, and what do you find is the best way to determine CBTO for your rifle? I'm also aware of the importance of magazine clearance.

Thanks, : )

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First you find out the maximum length your rounds will feed from the magazine unless you plan to single feed and then juggle the bullet length.

OK--I was out of town for a while and had a chance to fire my reloads that I did with the CBTO technique I got from the Berger book and I'm a little concerned that I may have not done it right. I did check all the loaded cartridges for accuracy and everything seemed correct--except the very first cartridge I chambered into my rifle seemed to have significantly more resistance to closing the bolt than my other reloads that I did by simply following the COAL listed in the hornady book. The bullets on the CBTO loads were seated long--and if I had to guess, they were probably too long and engaged the lands on closing the bolt. My gut feeling is that the bullet was seated too long and simply was being pushed back as the bolt closed. The cartridge fired and ejected normally and there were no signs of any over-pressure anywhere, but I nonetheless did not continue to fire other rounds.

I then repeated the "jamb" technique and that's when I noticed that the bolt upon retraction puts a significant sideways torque on the casing that almost certainly pulls the loose bullet in the casing (during the initial measurements) and only by using my finger to press down on the casing while retracting the bolt can I overcome this.

Another lesson learned.

Do the Sharpie method.

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Originally Posted by Fast1

Do the Sharpie method.

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That's the method I originally used.

Is a step by step needed ?

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Originally Posted by Fast1

Is a step by step needed ?

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be glad to hear what you have to say--if it's more than what I outlined above.

What round are You trying to figure this out on ?

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Originally Posted by Fast1

What round are You trying to figure this out on ?

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6.5 284 through my savage 111. One thing I still don't understand is that the Hornady book lists max COL as 2.8; while the Nosler book lists 3.310 and my cartridges COL are well under that. The Nosler book says it's specs are "true norma specs" but my 111 barrel clearly says 6.5 284 Norma on it. I'm stumped (it is possible. I guess, that the neck simply had some friction when closing the bolt on my test fire).