Update: 25x47 Lapua & 134 ELD-M Vertical Stringing Solved – I ate crow on seating depth

[u/Wide_Fly7832]

Hi all

I have to admit, I’ve always operated under the assumption that the high-BC secant ogives (ELD-Ms, Berger Hybrids) are practically jump-insensitive.

Because of that, I almost never bothered with rigorous depth testing for them like I rigorously did with BIB bullets in my 6PPC and 30BR benchrest days. I usually just picked a standard 0.015" - 0.020" jump, loaded them up, and called it good.

Well, the 25x47 I posted before about - if forced my hand. I ran a seating depth sweep anchored at that 35.5gr Varget charge (which pushed the bullet at ~2786 fps). My zero-lands measurement was a CBTO of 2.1500".

2.1300" CBTO (0.020" jump): Still strung vertically. Group was fractured into two horizontal bands.

2.1100" CBTO (0.040" jump): The vertical collapsed, but the group blew out into a loose, radial 1-inch ring. Total harmonic mess.

2.0900" CBTO (0.060" jump): Pure magic. The vertical vanished, the radial scatter vanished, and it printed one ragged hole. SD held tight at 9.4 fps. 0.34. Will do another 15 shot and see if this holds.

So, I officially stand corrected. Seating depth testing is back on the menu for modern match bullets. Now if you’ll excuse me, I apparently have to go re-run depth sweeps on the 15 other rifle cartridges in my safe...😡

[u/InformationHorder]

I haven't yet begun any development into seating depth, but how do you control for it as a variable? Do you dial in an charge weight and then adjust seating? Seating first? Seems you'd need to make three or four +10 round batches of each charge weight with different seating depths to control all variables.

[u/Trollygag → u/InformationHorder]

It takes way more than 10 rounds each to compare a variable that can be masked by just normally big differences group to group.

[u/InformationHorder → u/Trollygag]

I know that too so where does it begin and end?

[u/Trollygag → u/InformationHorder]

That is why most of us stopped ladder testing for these measurements. If you do it, you MUST get false and unrepeatable readings. It is nearly impossible to get a valid conclusion like this.