So I'm finally at point where I'm trying to find my "forever" load for my rifle. I even did a fire forming round of my new brass. All the components for my load is chosen because it's a very widely used. The final step is to find the optimal charge weight.

My question is about setting the COAL. Using my Hornady OAL tool, it shows for the sample bullet a COAL of 2.760" or so. The Hornady manual says 2.800". So if I were to target 10-15 thousandths less then that would make the COAL 2.740", which would be 60 thousandths less than the manual. Should I be concerned about this?

Load info: 308 Lapua brass, cci 200, varget, 178 eldm
Rifle: Savage with Shilen pre-fit

I am revising some of my procedures/process for relaoding and I tried this exact same sequence on a tikka t3 varmint in 223. The tool showed 2.320", manual for 68gr bthp shows 2.250" so I seated to 2.300". The additional 50 thousandths doesn't seem like a big deal and makes sense. I thought I heard that factory rifles often come with long throats and my 308 is an aftermarket barrel. I'm concerned 50-60 thousandths shorter is an issue.

Final bit of info, during the fire forming session I seated the same combination to 2.800" and found a charge weight that yielded a single 5 shot group of 2.00" at 300 yards. I've check several of my fir form batch an indeed all 2.800" loads would touch the rifling.

Have you determined length where the bullet just touches the lands? Do that first and then test in increments of .015. You may find your rifle shoots better than listed COAL (assuming it fits the magazine).

CBTO is determined by chamber.

After that, it is determined by magazine length.

Who cares what the manual says? They wrote that for SAAMI specs, which have broad tolerances. Make ammo to fit the rifle it is going in.

+1.

I know it's a rhetorical question but I guess I do. Only in that if my chamber requires it to be 50-60 thousandths less, I thought there might be issues because it's a lower case volume. But I guess I'm going to trust my gauge measurement and seat it 10 less.
I suppose if I'm concerned about the compressed load then the answer is just watching for pressure signs as I shoot. I'm still a newb and get paranoid that I'll blow my face off or ruin my rifle or something equally stupid.

Start low and work up watching pressure and velocity when seated deeper than published data, then again that is standard operating procedure. I will start off with one loaded to max functioning mag length (if I have a single shot it will have a falling block action not bolt). At ax mag length check it it is making contact with the lands. If not then try that length with a ladder. Just what has worked for me. A lot depends on the bullet as well some are more seating depth sensitive than others.

Here is a good article from Berger on how to determine the seating depth "sweet-spot". While it refers to their VLD bullets it hold true for most cup-n-core bullets. But note that monolithic bullets are different on what makes them shoot best.

Berger - Determining Best Seating Depth

Here is an excerpt:

Trying to find the COAL that puts you in the sweet spot by moving .002 to .010 will take so long the barrel may be worn out by the time you sort it out if you don’t give up first. Since the sweet spot is .030 to .040 wide we recommend that you conduct the following test to find your rifles VLD sweet spot.

Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

Shoot 2 (separate) 3 shot groups in fair conditions to see how they group. The remarkable reality of this test is that one of these 4 COALs will outperform the other three by a considerable margin. Once you know which one of these 4 COAL shoots best then you can tweak the COAL +/- .002 or .005. Taking the time to set this test up will pay off when you find that your rifle is capable of shooting the VLD bullets very well (even at 100 yards).

pertnear's test is good.

My Tikka actually shoots about the same at .010, .020, and .030 off the lands. It starts deteriorating at .050 and greater, but it isn't all that sensitive to jump. I've heard that 6.5CM in general isn't that jump-sensitive, so that may or may not apply to your loading. You kind of have to determine what your rifle and bullet combo acts like.

You should totally ignore SAAMI. Measure your chamber and load for it. Hopefully it fits in the mag. If not, decide if you want to test out other COALs or other mag options, or single load.

This