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Bullet seating depth vs pressure
dtknowles
Member Posts: 810 ✭✭✭✭
Ok, I have heard it both ways now, and looking for a better rule of thumb.
Rule #1 Seating your bullets farther out into the throat will raise pressures
Rule #2 Seating your bullets farther into the case will raise pressures
I think both statements can be true in different cases so to tell which one will be true in which cases.
In the one extreame where the bullet is seated out to almost touch the lands then seating the bullet a little deeper into the case (farther off the lands) will reduce the peak pressure.
How far off the lands do the bullets have to be to be at the point where pressures start to be insensitive to the distance off the lands.
On the flip side if the small difference in case volume from commercial brass to military surplus brass can cause pressure increase then a bullet seated deeper into the case will have the same effect.
which effect is more pronounced?
Tim
Rule #1 Seating your bullets farther out into the throat will raise pressures
Rule #2 Seating your bullets farther into the case will raise pressures
I think both statements can be true in different cases so to tell which one will be true in which cases.
In the one extreame where the bullet is seated out to almost touch the lands then seating the bullet a little deeper into the case (farther off the lands) will reduce the peak pressure.
How far off the lands do the bullets have to be to be at the point where pressures start to be insensitive to the distance off the lands.
On the flip side if the small difference in case volume from commercial brass to military surplus brass can cause pressure increase then a bullet seated deeper into the case will have the same effect.
which effect is more pronounced?
Tim
Comments
As long as the bullet is not jammed against the rifling, i.e. it is off at least 1, 2, 3/1000 of an inch I think you are fine.
Conversely, if you jam a bullet down so it crunches the powder charge and is seated way low, pressure will rise- how much depends entirely on load. Seated over a charge of bulky pistol powder with a slow burn rate in a moderately sized cartridge like a .38 S&W with a wadcutter it may be just fine; do this in a magnum rifle case already at a max load and you may be asking for trouble (especially if you do it repeatedly).
OK for tough hunting bullets, but I am a target shooter and I jump Sierras about .012" and jam JLK or Berger VLD into the rifling by the same amount.
Vihtavuori says the change in case volume of a bottleneck cartridge by different bullet seating has little effect in and of itself. It is the jump to the rifling that counts.
The two rules you stated are exactly that. Two separate rules. If you reduce case size by seating deeper pressure will rise above normal. If the bullet starts with pressure against it from the lands pressure will rise above normal. I think the latter causes the bigger increase so care must be taken so as not to overpressure. The former can be bad only when you have a compressed load with too fast of a powder. I load 300 win mag and use a compressed load with H1000 behind a 208 Hornady. bullets seated to the lands...not in them. Pressures are fine. If I were to use VV N170 or 4831 there would be issues.
Anyhow, I make sure when reloading or working up a load with the bullets in the lands to work up more slowly than I normally do. As always carefully watch pressure signs and jumps on the chrony for a given load. Such as each .2 gr. increment you get +30 fps. Then the next group you get 80 fps increase. Deep seating I usually work up normally, but always pay attention to jumps on the chrony.
Can you give me a better pointer for the Barnes Bullets article you mentioned. I went to a Barnes bullets . com and there is a lot of stuff there but I did not see what I think you were pointing too.
Thanks
Tim