how far to seat off powder?

S

Shadow9mm

New member
Have been doing some reading as far as seating depth in bullets. As I understand it increasing the seating depth (shorter OAL) or increasing the powder charge can greatly increase pressure.

Is there any reason you can't work up a load based on powder to bullet base?

My 9mm measurement for 124g Hornady XTP over HP-38

Measure powder lvl to case rim. 0.308
measure bullet 0.570
measure case 0.741
Loaded OAL 1.148

With these numbers I figure the bullet had 0.163 seated into the casing. not really sure how much it needs to seat.

This gives me a final number of 0.145 between the powder and bullet

I know compression is bad, but is there a set distance or minimum distance between powder and bullet that I can go by?
 
Seating depth seems to be the key.
As long as the base of your bullet is in the same place as the bullet used in the data, you shouldn't have to worry about how far off the powder you are.

With many powders it's fine to have some compression, like Unique or Blue Dot, makes for more uniform ignition.

But with HP-38 you'll never be compressed in 9mm, it's too dense.

Set your OAL's to match your data.
 
Especially in handguns, I'd suggest seating depth and compression may have occasion to overlap,,,and when they do,go ahead and think about it.

Meanwhile,I think you are on a correct understanding that its the depth of the bullet base that influences pressure. While component substitution ,as in equal weight and construction of bullets,requires caution and a work up,

your calipers and a little math will allow you to seat he bullet base to the same depth.

Your load manual will let you know if powder compression is an issue. I don;t mind a little. I know a double charge won't fit
 
Increased airspace inside the case lowers peak chamber pressure and conversely the lack of air space increases peak chamber pressure in a very predictable fashion. Compression tends to increase pressures but some powders do not seem to behave well compressed.

I use Quick Load software. It models what goes on inside of the cartridge and predicts pressures and velocities. One handy feature of Quick load is that it calculates percent of case fill. When compressed the number is above 100. Sometimes I play with seating depth and/or the value for case volume in the software specifically to see how much the results change for a given change in free space.

I generally prefer a small amount of free space as it tends to be more forgiving (smaller variations in the results) with respect to variations in case volume.

Note that with a lot of free space, and variations can be caused by powder location (front of case vs back of case).

Unless you are using all matched brass, you are likely to have small small changes in cartridge internal volume from round to round. With compression or very little free space, these changes make a bigger difference in peak pressure and velocity.

I my experience, mixed 9mm brass is about as bad as it gets for variations in internal volume. I use range pickup brass for the 9mm, but only 3 headstamps (Blazer, FC and Speer) that combination seem to give me consistent results.
 
Last edited:
Shadow9mm,

The problem with what you propose is that the powder's bulk density is not very consistent from lot to lot. Look at Accurate's site, and you see their powders vary as little as ±2.2% to as much as ±5.6% from lot to lot. What matters is the amount of space under the bullet that a certain amount of powder mass starts burning in. The mass determines how much gas will be made, while space determines what pressure that much gas will create, though it has to allow for the extra space that appears as the bullet starts moving (expansion). But the distance from the top of the powder pile to the bullet base will never produce a consistent enough result. With pistol powders, a ±5.6% powder variation could produce upwards of a 30% difference in pressure from one end of the range to the other.
 
You must log in or register to reply here.
Back
Top