seating depth issue

[zpyoung]

Currently, I'm shooting a 6 Creedmoor that I had a custom built. I have shot around 500 rounds through this gun. After running out of bullets, I decided that I should purchase another 500 counts of Berger Hybrid Target 105 Grain Hollow Point Boat Tail. Before I was only buying 100 counts at a time This new batch is the same as what I was shooting with before. In the past, I have noticed that new batches of bullets will require a new seating depth.

When I picked up the gun from the gunsmith for the first time he was nice enough to help me determine the correct seating depth. His method was different, and he didn’t use the Hornady OAL gauge. He basically determined it by trial and error by feeling the amount of pressure that was required to close the bolt on the gun with a dummy round in the chamber. This has produced very accurate ammo up to this point.

With the new batch, I started with the Hornady gauge and determined that my max Cartridge Base to Ogive dimensions were 2.300, 2.3055, 2.3045 and 2.3045. Then I calculated an optimum seating depth of 2.28. I also tried a different bullet from a different batch that I had left over and got similar dimensions as above.

With the gunsmith, I got an optimum seating depth of 2.2275.

At this point, I can’t explain why the two methods are resulting in such a large difference. With my method using the Hornady OAL gauge, my COAL is too long. I can barely get the ammo into my magazine and I can’t get a dummy round to eject because it’s too long.


Should I just keep running the original optimum seating depth?

Could I have had enough throat wear to cause this difference already?

The other issue that I should mention is that I bump the shoulder back too far on about 200 rounds. Normally I will bump the shoulder back to .002 but this batch I did it at .020. I figured this would be ok to shoot. The only issues with this were that I had a few misfires and a few pierced primers.

Thanks
Zach

 

[ShtrRdy]

The Berger 105 Hybrid is said to e less sensitive to seating depth. Therefore I would just do what you've been doing with the 105 Hybrid and see if it works as expected.

 

[Country]

Wo up there ! If you bumped the shoulders .020 ( 20 thousandths ) don't shoot them the cases will strtech to hell . Use a large neck expanding mandrel and then partial size the length of the neck back down until the case is a crush fit in the chamber . You had miss fires because the case moved in away from the firing pin . Pieced primers because then the case slammed back onto the pin . Those cases are over stretched now and prone to early failure at the web . I would dump them .

 

[zpyoung]

Thanks for the replies. I still can't figure out why the spread between the two methods is so different.

 

[Deleted member 10043]

Make the Hornady case neck is not too wide causing the case to chamber short. Like try and close the bolt on the case.

 

[pbatesaz]

FWIW I agree that the cases bumped 0.020” should be discarded. You will likely have to limit your COAL to what your mag will accommodate, unless you want to single feed rounds.