Group size

[PaLuke]

Hello. I loaded some 30-378's today and trying a different load I seated the bullet(208 gr A Max) .010 deeper and my groups went from 5/8" at 100 yards to almost 2". I shoot 5 shot groups with 104 gr. of Retumbo. Has anyone else ever had results like this? I load for different rifles and never had this much change with such a small adjustment. Thanks for the replies. PaLuke

 

[Mikecr]

With any gun I've had, I could find a seating point where very small changes produce very big results. I'm absolutely convinced that bullet seating is as big or bigger than primer changes and way bigger than ANY amount of powder change.
Afterall, you could not open your groups from 5/8" to 2" with powder alone. Right?

With this, it never made sense to me that people would just pick any ole seating while load developing with powder, and then attempt to change seating -while expecting improvement.
That amounts to setting a coarse knob at a random position, then adjusting a fine knob for pseudo-tune, then adjusting the coarse knob(as though a finer adjustment).
Backwards..

 

[PaLuke]

I completely understand what you mean. I just never had a seating depth change so small open up a group that much. I see now its very possible. Thanks for the info

 

[Greyfox]

That is a pretty big change for only a .010" difference in seating depth, particularly with an AMax. How far fron the lands is your load that produced the 5/8" group? It is a long shot, but is it possible that your increase in seating depth pushed you just far enough into the lands to create a high pressure stituation that degraded the accuracy?

 

[EddieHarren]

"....and trying a different load...." You actually changed 2 of the dynamics in your ammo. Different load and different seating depth. If you want to know which one caused the groups to open up you can only change 1 factor art a time.

 

[PaLuke]

hi again.. Reading my first post now I realize I should of given more specs on my rifle and maybe this would help seeing my surprise at the results. My rifle is a stock Accumark with a load of freebore. My magazine is 3.8" and to clear a live round from the chamber it needs to be 3.5". For me to reach the lands with the bullet the OAL needs to be 4.150" The lands measurement was gotten with blackening the bullet and putting it in a case and pushing it into the barrel and the measuring the OAL. That's what surprised me about going to 3.490" and the groups going to 2". I guess with all that free bore bullet seating depth has as much meaning as with a rifle with regular dimensions. Im going to go back to my original load and call it quits.. Thanks for the replies

 

[Gene]

Changing the load and/or seating depth results in different pressure levels which in turn changes velocity and terminal results.

 

[RTK]

Changing the load and/or seating depth results in different pressure levels which in turn changes velocity and terminal results.

Bingo, and the harmonics of different velocities seem to make the biggest difference to me. I can run two different powders but at the same velocity and have very similar results.