Velocity change with seating depth

[woodnut]

I know seating depth will change pressures, accuracies, and some velocity changes. But I am having a hard time understanding a 150 fps change ! Exact same load, only seated bullets for a longer COAL but still without touching the lands. MY 6x47Lapau load of 38.0 gr. of H4350 and a 108 ELD went from a comfortable 2950ish to av. 3108! Accuracy and pressure signs appeared to be the same. I will load more to a slightly deeper depth, with 38.0gr, and with 37.5gr with both depths to see what difference may show. I did ladder the cartridge with 37.5 to 38.5 with pressures on a hot day with 38.5 gr. to be too much. Cronoed both the 38.0 loads on cool days of 65 degrees and found the changes in vel.
But has anyone had experience like this and what can you tell me?

 

[Canhunter35]

I'd be very careful with pushing your bullet out longer while using the same near max powder charge. You should work up to the max load at your longest coal then back the bullet into case. But yeah, being closer changes the spike in pressure probably sealing the bullet into the rifling faster with more pressure behind it. My guess anyways

 

[woodnut]

I just loaded some with 37.5 gr, both long and short COAL. Also 3 with the 38.0 load and short COAL length. Now all I need is a calm and warm day.....next Spring???

 

[Gord0]

Have you verified that it wasn't your chrono? Seems like a huge swing unless you were into the lands and didn't realize.