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OAL vs CBTO Illustration

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OAL vs CBTO Illustration
« on: September 21, 2018, 11:22:37 PM »


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For what it may be worth to some members here, while sorting out a new-to-me .223 AI, I've come up with a small sampling of throat location examples in a given rifle according to different bullets. I thought this may serve as a decent example of the differences between overall length (OAL) of a loaded cartridge and cartridge base to ogive (CBTO) determinations that we hear so much about, yet may still be a bit foreign to some.

We always see reference to OAL in loading manuals, and these are usually determined by magazine limitations and/or recommended seated lengths by the manufacturer for a given bullet. This doesn't often reflect what dimension an individual rifle's throat location is, according to rifling engagement on the ogive of the bullet, relative to the base of the cartridge (or more properly, the face of the bolt when firmly engaged with the lug abutments of the action).

Following are a few photos that some may find useful. I'm sure those 'in the know' will forgive the fact that some cases are formed .223 AI cases & some are still straight .223 cases. I assure you, all are properly sized according to the headspace in my machine.

The bullets: 40 Berger FB, 50 Nosler Ballistic Tip, 55 Speer TNT, 55 Hornady V-Max, 69 Sierra Matchking, 75 Hornady ELD-M. I just realized I didn't include the 68 Hornady BTHP Match in this photo. Oh well...

Photo simply showing each bullet's description.

Photo showing OverAll Lengths of each bullet according to the engagement of their respective ogives on the rifling.

Photo of each loaded cartridge with accompanying (to the left) bullet to illustrate the difference between OAL and equal distance to rifling (or lands, as some call it) engagement of different bullets.

Here you can see the differences that OAL lengths & CBTO lengths play. Magazine limitations, bullet shank engagements with the case neck, & powder space encroachment all have the potential to be considerations with you and your rifle, no matter the cartridge or bullet of your choosing.

I've done this for many years and have never really considered outlining any of it until now. I hope it has some use for someone out there.


Re: OAL vs CBTO Illustration
« Reply #1 on: September 30, 2018, 07:04:05 PM »

Jeff M

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Well done. This is something all new rifle reloaders should read.
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