This was sent to me by Marshall, a contributor here, and I wanted to send to all because it shows this velocity/pressure crossover point or the knee of the curve so well.
Velocity is lower than expected but several things could have contributed to that, one of which may simply be that the powder isn't the best for the cartridge/bullet combo.
Also this 20" barrel vs 24" for the data from Barnes plus the longer bearing surface and greater friction of Barnes bullets typically will give lower velocity.
I worked on some Barnes 270gr TSX in my rifle the last two days. Based on some comments you've sent this way I've tweaked my ways.
Since I haven't used Barnes and their powder charges and selections are so much different than everyone else's data I figured I use theirs.
I went out with the following charges in 4007ssc just to chart velocities:
79.6 2574fps 19fps increase
79.9 2586fps 22fps increase
80.2 2610fps 24fps increase
80.5 2631fps (their max charge) 21fps increase
80.8 2637fps very small gain, 6fps
81.1 2637fps no gain
Barnes TSX 270gr
My velocities were about 200fps slower than their data, that's why I went above their max charge. However, my increases didn't show a gain in FPS like the increments prior to their max. So I figure their max is a good number.
I had no signs of excessive pressures. Primers looked fine and case head wasn't tattooed by the bolt. The body of the case's measured the same on all brass. Only a .002 difference in case length between light and max charge.
I went home and loaded five samples each of 80.5, 80.2 and 79.9. They all shot well but the 80.5 grouped a tight .57", that's the best group ever out of my rifle. First shot was a cold clean barrle and the other 4 are touching. The grid is .75"x.75"
squares. Previous zero was 300 grain Game Kings.
The question is, why so much slower than their data. Nosler's info is right in the ball park when I follow their data.
Do you think my chrono is tits up?
Different length barrel causing a loss?
Their bullets are quite longer than equal weights from the competition and seated deeper into the case. Is that the reason for differences in powder choices and different amounts of the same choices?