CBTO and Max COAL - 6.5 Creedmoor

[Rathmatik]

Howdy,

I recently acquired a Bergara B-14 HMR in 6.5 Creedmoor, and also got the Hornady OAL gauge and comparator setup.

I just bought some 143gr ELD-X bullets to make some precision and hunting loads. I tested 5 bullets, and came to a consistent 2.243" CBTO. This measures to ~2.890" COAL.

The HMR has a AICS magazine, which has a max allowed COAL of 2.860", so clearly I need to reduce my CBTO by at least 0.030" off the lands. I figured I might start with 0.040" off the lands and go from there. This would give me 2.203" CBTO and something in the ballpark of 2.850" COAL.

Now, my question is this. Since Hornady defines the Max COAL for 6.5 Creedmoor to be 2.825", am I required to remain under that length for any reason? Or is that just the SAAMI specification for these short actions?

Thanks!

 

[Supersubes]

No requirement at all, test away.

 

[Rathmatik]

No requirement at all, test away.

Cool deal, just wanted to make sure I didn't miss something from all this CBTO talk.

 

[Ryridesmotox]

Cool deal, just wanted to make sure I didn't miss something from all this CBTO talk.

I used to use tipped bullets as well. You can load a non ballistic tipped projectile a cunt hair longer. Just food for though, if you want to make up some of that seating depth

 

[NotBotvinnik]

Hi There-

I’m also in the process of taking some base measurements to work up a load for my 6.5CM Bergara B14 HMR. Just like you I started with the standard Hornady OAL gauge w/modified case. I’m loading once fired Hornady brass bumped back .020” (fired by my rifle from factory Hornady ELD ammo) and Lapua 139gr Scenars.

Now- I don’t trust the Hornady OAL gauge as it came from Hornady. The plastic rod used with the gauge body made for inconsistent measurements with its mushy feel and possible bending. I’d take seven measurements the exact same way and write them down. The numbers were all over place. I had nothing to work with. Seven measurements, all taken as uniform as possible with an extreme spread of 0.080”. Yup- that’s eighty thousandths. It had me literally about to pull my hair out.

I started thinking the flimsy plastic rod in the gauge may be the culprit. I had an extra Pro-shot stainless steel cleaning rod laying around and measured it’s diameter with my micrometer. As luck would have it it’s the same diameter as plastic rod. I immediately got to work cloning the plastic rod.... in stainless steel.

Took seven measurements with my new and improved Hornady OAL gauge and presto! Almost zero variation!!! In fact my last two measurements were identical to the half-thousandth. Now I finally had faith in my numbers.

What I got-

2.1910” max CBTO

2.1710” base to ogive seating -0.020” OTL

2.7875” COAL finished cartridge

I don’t know if any of that info is of any use to you but seemed like a good thread to post it in.

Cheers

 

[nn8734]

Cool deal, just wanted to make sure I didn't miss something from all this CBTO talk.

Just curious, have you ever shot any factory Hornady ammo that uses the 143 eldx?

If so, how did it group?

If it grouped tight consistently I’d measure the BtO of an unfired factory round and start your testing with bullets seated to that BtO measurement. Then adjust as needed. Most of these hybrid type bullets actually like some jump...

 

[NotBotvinnik]

Please disregard the numbers in my lengthy above post- I forgot to account for the length of my measuring tool- Jeeez

COAL 2.846
CBTO 2.240
Those are my numbers touching the lands

seated back 20 thou OTL
COAL 2.826
CBTO 2.220

Sorry for the mixup-