Thread: Bullet Seating depth and pressure (6.5 Grendel)

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 ?

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.

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.