Will chamber pressure be significantly increased or decreased by the bullet seating depth? i.e. closer to lands increase; farther from lands decrease. Thanks in advance.
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Will chamber pressure be significantly increased or decreased by the bullet seating depth? i.e. closer to lands increase; farther from lands decrease. Thanks in advance.
Seating the bullet deeper will increase the pressure. As to if that equals significantly, perhaps as there are other deciding factors.
The are two factors working in opposite directions.
1. As the bullet is seated deeper, the effective initial powder space decreases, raising pressure. This can be critical in small-capacity cartridges where deep seating can seriously reduce available space.
2. With tough bullets, seating deep enough so they can acquire some momentum before encountering the lands will normally reduce peak pressure somewhat.
In medium-capacity rifle cartridges with ordinary thin-jacketed, lead-core bullets and powder charges at 95% or less loading density, neither factor is likely to have enough effect to notice any difference in pressure - although there may be significant variations in accuracy due to yaw in the throat with deep seating, especially with VLD designs in oversize throats.
Here is what I've read on the matter.
If you are talking about seating the bullet deeper into the case, pressure will increase because of decreased volume inside the case.
If you are talking about the bullet seating towards the chamber, you have three scenarios.
A. Seating the bullet in the case so that the bullet is just in front of the barrel grooves.
B. Seating the bullet in the case so that the bullet is just touching the front of the grooves.
C. Seating the bullet in the case so that the bullet is started into the barrel grooves.
A would produce no increased chamber pressure but is considered the least accurate projectile on target.
B would produce some increased chamber pressure but is considered more accurate projectile on target.
C would produce more chamber pressure than B would. I don't remember if bullet accuracy is more than B.
Of course, if you are using max powder charges, pressures go way up. Also, one must consider the size of the bullet and overall cartridge min/max lengths.
I always use middle range powder load for my M1 using IMR 4895 and I trim cases to min lengths and retrim when case lengths after resizing reaches max.
Handguns is where it can matter more. The bottleneck rifle cases not so much as long as you are within a reasonable range for OAL.