Why Does Velocity Drop The Shorter You Seat The Bullet?
- Thread starter sig2009
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It's supposed to increase just as you said. The deeper you seat the bullet the less volume the case has, thus the more pressure made and higher velocity. Are you sure all your variables are the same? Temp?
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There's more to it than case pressure. There's also pressure in the chamber wall sealing from the case neck.
then there's pressure when the bullet hits the lands and seals the bore.
There's always let offs and pressure build up going on from different actions.
At this point it's only an observation; nothing you can really do to apply towards load development. You do what the target says and the velocity you get is what you get.
then there's pressure when the bullet hits the lands and seals the bore.
There's always let offs and pressure build up going on from different actions.
At this point it's only an observation; nothing you can really do to apply towards load development. You do what the target says and the velocity you get is what you get.
I ASSume you are using the same powder charge. (For the boys and girls at home, reading this like me.)
I'd say length of the throat-freebore a big player here, if it is a real long throat, pressures generated at say .010" off will not be achieved again until you are .070-.100" off, with a drop in between.
Now on an 223 AR load, say a 75gr bullet at mag length,, you start pushing the bullet into the case, you're building pressure fast.
Make sense?
Edit: Once the boattail junction of a bullet gets inside the neck shoulder junction, things go up reasonably fast also.
Now on an 223 AR load, say a 75gr bullet at mag length,, you start pushing the bullet into the case, you're building pressure fast.
Make sense?
Edit: Once the boattail junction of a bullet gets inside the neck shoulder junction, things go up reasonably fast also.
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Assuming all other variables are identical and your only adjust seating length, the further you seat in the more pressure you should generate creating higher velocities... Were you changing other things?
I think the exact combination is important, theres no rule. Mostly though, I think the phenomenon of increasing pressure by deeper seating is overstated. Most of us are loading in a fairly small window near the lands, as in slight jam to say .050 off. As best i can recall, Ive always found pressure dropping as I move away from the lands in that realm. I chalk it up to the bullet getting a run at the lands, as opposed to the bullet stalling out on the lands at the point of engraving, as the pressure builds behind it. Even If your combination was building pressure the deeper you seat, adjust the load, as you would if you were developing more pressure than you like, while moving closer to the lands.
I chambered a barrel in saami 338-06 A-square recently. Long freebore, with no reasonable way to touch the lands(not enough bullet in the neck). Ive played with seating depths from about .050 off to .300" off, with no significant changes in pressure. This is a slightly unusual cartridge with its relatively small capacity to bore area. I also did a creedmoor barrel that shows virtually zero change in pressure between .010 off and .040 off. I tested it in .010 increments, and those four loads averaged within 10 fps of one another. On the other hand, in the small window between .010 off and touching(never tested jam) I see pressures building quickly. Not unusual in my experience, and why I usually load off the lands. I mention these two barrels only because they're recent.
I chambered a barrel in saami 338-06 A-square recently. Long freebore, with no reasonable way to touch the lands(not enough bullet in the neck). Ive played with seating depths from about .050 off to .300" off, with no significant changes in pressure. This is a slightly unusual cartridge with its relatively small capacity to bore area. I also did a creedmoor barrel that shows virtually zero change in pressure between .010 off and .040 off. I tested it in .010 increments, and those four loads averaged within 10 fps of one another. On the other hand, in the small window between .010 off and touching(never tested jam) I see pressures building quickly. Not unusual in my experience, and why I usually load off the lands. I mention these two barrels only because they're recent.
