COAL vs CBTO

DallyB09

FNG
Joined
Nov 30, 2024
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Rookie question here so go easy. I understand the measurements clearly, what I don’t understand is why books list COAL. Is it a CYA so that ammo will run in any chamber?

The COAL can vary just based on tip of the bullet, even factory ammo has a pretty large variance in COAL.

*things I think about while I lay awake at night 😂😂
 
Both are usable information, ogive is for working with the lands, coal is for action and magazine length if you want to look at it that way. I'll give you a example of a situation I had recently. Built a 25prc on a short action, this particular action can handle 3.150" coal. Combined with um magazine or a baney box I have atleast 3.1" of usable space, with a 135 eldm I'm just under 3.1" coal to keep the bullet off the lands without touching and the boat tail out of the donut area. Next step is seat a bullet and drop it into the chamber and figure out when it touches that is a cbto measurement I'll keep for future work. So when it comes to putting a gun together around a cartridge I use coal, when reloading I use cbto. Hope this helps.
 
Because everybody’s comparator tools are different. If they listed CBTO and a guy has a perfectly exact dead nuts to the .0001 comparator and the next guy loads with a Hornady comparator then numbers are gonna be further apart then COAL even with bullet variance.

I don’t even give friends BTO numbers unless we have matched up our comparators and figured out the difference so we can add/subtract off what the other guys set measures.
 
As mentioned above. I use CTBO because my Tikka's and Browning Abolt are not limited to mag length rounds. However, both of my Rem 700 adl's are, so I measure COAL for those to insure they cycle without any issues.
 
I'm a beginner reloader. I use CBTO for measuring and adjusting jump, since it's a repeatable metric. I use COAL to confirm the rounds fit in the mag and feed at my desired jump.
 
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