Thread: Bullet Seating depth and pressure (6.5 Grendel)

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

Gidday Pete - I ran these loads in Quickload to see what it said . . . which is only an indication but sometimes helps to understand what is happening.

Q/L doesn't have ADI 8208 but it shows IMR 8208 XBR - and since all these are marketed by Hodgdon one might "assume" they are the same - but no guarantee.

At 27.7gn of IMR 8208 XBR you should be getting about 2530 fps from a 22" barrel (I'll recalculate it if you let me know the actual barrel length), and pressure at 2.25 COAL is indicated at 53,800 psi. At a COAL of 2.26 the pressure indicated is 53,300, so bugger all difference. The Leade design and free bore will also be a factor. Just to fully disclose, the SAMMI max pressure for the 6.5 gendel is 52,000 psi so your loads are already "up there" - at 28.5gns the pressure is indicated at over 58,000 psi

Originally Posted by Tentman

Gidday Pete - I ran these loads in Quickload to see what it said . . . which is only an indication but sometimes helps to understand what is happening.

Q/L doesn't have ADI 8208 but it shows IMR 8208 XBR - and since all these are marketed by Hodgdon one might "assume" they are the same - but no guarantee.

At 27.7gn of IMR 8208 XBR you should be getting about 2530 fps from a 22" barrel (I'll recalculate it if you let me know the actual barrel length), and pressure at 2.25 COAL is indicated at 53,800 psi. At a COAL of 2.26 the pressure indicated is 53,300, so bugger all difference. The Leade design and free bore will also be a factor. Just to fully disclose, the SAMMI max pressure for the 6.5 gendel is 52,000 psi so your loads are already "up there" - at 28.5gns the pressure is indicated at over 58,000 psi

Thanks Tentman for doing that.
That's interesting since ADI list 28.5gr. max charge but for the Sierra 123gr. BTHP (not the 123 SST) it would be interesting to run with the Sierra bullet as well.
Were down to an 18 inch barrel and using Federal 205 small rifle primer / hornady brass if any of that makes difference.

Interesting that quickload theoretically shows higher pressure with a slightly deeper seating - I would assume (have read) thats because of case capacity being reduced ?
This is a common load and lots on here have had best results around the 28gr. or slightly over mark.

Yes, seating deeper increases pressure. But 0.020" won't cause a safe load to become dangerously unsafe.

Seating longer can also increase pressure. Without a run up to the rifling, more pressure builds up in the chamber before the bullet gets moving.

My load is 27.6gr of BM8208 with the 123gr ELD-M. I get faint ejector marks too. I've gone up to 28.5gr and it was very accurate but the bolt was starting to get sticky.

Originally Posted by PommyMcPomFace

Yes, seating deeper increases pressure. But 0.020" won't cause a safe load to become dangerously unsafe.

Seating longer can also increase pressure. Without a run up to the rifling, more pressure builds up in the chamber before the bullet gets moving.

My load is 27.6gr of BM8208 with the 123gr ELD-M. I get faint ejector marks too. I've gone up to 28.5gr and it was very accurate but the bolt was starting to get sticky.

That's interesting since we've both come to almost the exact same load. Is that what you are using on a regular basis ?
My thinking is that I am obviously very near max pressure and if seated close to the rifling without a big jump it is increasing the pressure, seating a smidge deeper must help give it a a "run up" (like that) as you say and very slightly reduce the pressure. But if continued to seat deeper and deeper there will come a time when case capacity has decreased so much and you have a compressed load and high pressure again ?

You're very close to max with that bullet+powder.
ADI 8208 = IMR8208XBR
Increasing seating depth can cause dangerous pressure increases with certain powder/bullet/temp combinations. Always use the stated seating depth for a load or, if not stated, 2.260in (57.5mm).

There's a Grendel specific site which can tell you more than you'll ever need to know about the 6.5 Grendel.
Good bunch of guys for the most part.
It has user load data if you're serious about reloading for the Grendel.
Welcome to the 6.5 Grendel Forum : : The World's Tier One AR Cartridge
Check out the reloading forum and begin with BFT's Lengthy List of Loads.

The Q/L results with an 18" barrel - 2425 with 27.7gns. Velocity with the Sierra 123gn at 2.26 COAL is the 2430 and pressure just a tad higher - the same for all intents and purposes.

You don't say what brass you're running, but it can vary a lot too between brands in how it responds to pressure.

It was. My Grendel has now been mostly relegated to <100gr bullets at higher velocity for varminting, as I have another rifle or two to use for long range.

I don't think quickload is as good at predicting pressures and velocities in the Grendel as it is with more common cartridges like 223 and 308. But it still gives a good indication:

If you know your bullet is up against the lands, quickload instructs you to add 7200psi to the "start pressure" and when you re-run the calculation after factoring that in, the 53k psi load that @Tentman demonstrated turns into a 63k psi load. Going the other way, away from the lands, it reckons you'd have to seat the bullet all the way down to 2.100" OAL to get the same rise in pressure.

The Grendel Saami pressure is low for Gas Guns, bolt actions can handle a little more depending on brass.

I found 28 grains of 8208 to be ridiculously accurate in my Grendel Proof upper

The SAAMI pressure was relevant at the time to AR based gas guns of which the round was designed for. Also the SAAMI chamber has a stepped throat to lessen the pressure spikes, there is plenty of info about this on the aforementioned website. There are varying chamber designs now being used by firearms manufacturers and it would be interesting to know which design Howa are using.
I have found the Grendel to be the more touchy of all my rifles in regards to pressure when load developing.

Between starting this post and finishing my coffee, Greg has reported similar comments

Originally Posted by Tentman

The Q/L results with an 18" barrel - 2425 with 27.7gns. Velocity with the Sierra 123gn at 2.26 COAL is the 2430 and pressure just a tad higher - the same for all intents and purposes.

You don't say what brass you're running, but it can vary a lot too between brands in how it responds to pressure.

Thanks again, were using Hornady brass

Originally Posted by PommyMcPomFace

It was. My Grendel has now been mostly relegated to <100gr bullets at higher velocity for varminting, as I have another rifle or two to use for long range.

I don't think quickload is as good at predicting pressures and velocities in the Grendel as it is with more common cartridges like 223 and 308. But it still gives a good indication:

If you know your bullet is up against the lands, quickload instructs you to add 7200psi to the "start pressure" and when you re-run the calculation after factoring that in, the 53k psi load that @Tentman demonstrated turns into a 63k psi load. Going the other way, away from the lands, it reckons you'd have to seat the bullet all the way down to 2.100" OAL to get the same rise in pressure.

Ok, thats good to know and answers my question, theoretically a huge jump in pressure if loaded out to the lands. I don't exactly know for sure where the lands are on this rifle yet, I just loaded a dummy round at pretty much maximum magazine length (2.260") and made sure we it was feeding ok and not getting stuck in the rifling. But I suspect we may be close. Clearly you have to seat a bullet very deep (2.100") at the other extreme to get a pressure spike.

This is why we are keen to see if we can get this load working at safe pressures of 27.6 / 27.7 gr.
All shot by my son and the 27.7 was a 4 shot group all touching at 100 yards. Stock standard Howa Mini.

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 !

Thanks for your comments @Bagheera and nice shooting. Sure is an accurate little cartridge, that 16 inch barrel must be handy. We went down to 18 inch and I was reluctant but glad we did.

I do have a bullet comparator for the Grendel and have taken notes on Ogive but since we are limited to maxazine length I assumed we would not be close enough to the lands to get overly concerned (thinking only from an accuracy stand point), but I was not aware of the effects it would have on pressure.

It would seem that most people have had success with ADI 8208 up around that 27.5 - 28 gr. mark and that would appear right on the edge of a pressure spike.