Thread: Bullet Seating depth and pressure (6.5 Grendel)

Originally Posted by Pete_D

Background - We've been developing a load for my sons 6.5 Grendel and have found an accurate load at 27.7 gr. of ADI 8208 with 123gr. Hornady SST.
(We've used load data from ADI which lists a max load of 28.5 gr. 8208 but they do not list the SST specifically rather a 123gr. Sierra HPBT so we ran with that.)

All our loads so far have been at 2.260" COAL, but when I loaded up some more rounds of 27.7gr. to verify last weekend, whilst setting up the seating die I slightly over seated the first round closer to 2.250" COAL and I thought no worries, I will do three like that and use them for warm ups.

Well, all shot good with no noticeable pressure signs, no hard bolt lift or flattened primer etc. But after reviewing the brass back home I was surprised to see some of the loads show a very (very) faint ejector mark on the case head. I did not notice it whilst shooting and only because I used a magnifier glass back home.

This is where it gets interesting because the three rounds loaded at 2.250 do not show any ejector marks at all.

Question to those who are still with me:
1. Before I load some more up to verify is what I am seeing pretty typical that reducing seating depth reduces pressure ?
2. Are very faint ejector plunger marks a worrying sign to over pressure. (the only other rifle I have loaded for is a Mauser so you would never know)

Cheers guys

So, your shorter rounds show less pressure (as shown by ejector marks).
That's what you'd expect if you were starting to touch the lands with the 2.260" seating.

The key dimension is not COAL because that depends on bullet shape.
It is the case base to ogive (CBTO) which directly measures how close you are to the lands.
The SAAMI spec for chambers on 6.5G is 1.7025 to 1.7045".
For COAL (loaded rounds) it's 2.135 to 2.260.

I did a safety check by doing a ladder with mine.
I'm using Nosler 120gr ballistic tips so the bullet shape is different.
I tested up to 1.7215 CBTO and it seemed safe but have settled on 1.700" because if they are longer I can't eject a live round. It's a claw type extractor not plunger. I don't know if I can even get ejector marks ? As best I can tell, this is 0.020" off the lands. It's a chamber cut by Scott Trail and appears to be very close to SAAMI spec.

Interestingly, I am using 27.5gr of BM8208 too. they are federal GM205M primers and Lapua brass. I've tested up to 28.5gr but that seemed to be a compressed load because the CBTO and COAL came out longer despite using the same seating die setting so it sort of rebounded or something, which I think is a sign of a compressed load. Anyway I'm not doing that.
This shoots pretty well but one day I'll try longer loads again and a drop tube to see if I can cram more powder into the case. I'm only using a 16" barrel and need every fps I can get.

Here are a couple of 5 shot groups. I don't know that I'll be able to recognise if I improve on those accurascy wise.
The top one is without the suppressor to check where it goes. No excuses for the loose shot - it makes a racxket and I lost concentration !