Powder volume vs accuracy

[azrocks]

Intuitively, it seems that if all other parameters were optimized, you'd get the most consistent powder burn by filling the case up just to the base of the bullet.

Is that the case (no pun intended), or am I off the mark?

EDIT: For the engineers out there, I guess I mean precision, not accuracy

 

[Galil5.56]

I sure like, and prefer a very high load density, say 95%+, and just shy of compression in rifle loads. Especially so with any ball propellant I use.

 

[Orcon]

I see what you're getting at, sublimation rate for a given volume but how do you quantify that volume in a repeatable way? And would it be significant when weighed against variations like true bullet diameter, seating depth or primer intensity?

 

[Galil5.56]

I see what you're getting at, sublimation rate for a given volume but how do you quantify that volume in a repeatable way? And would it be significant when weighed against variations like true bullet diameter, seating depth or primer intensity?

Well, think if you have a a 50% load density. Aim up, and the charge is against the case-web. Down, it's on the bullet. Horizontal, it's like a trough filled up, with a mirror opposite of space above it... Not exactly the consistency you can achieve with the other variables you mention, in all conditions. I have however, used fillers for very light gallery type charges. Ball propellant, especially the 'slower' ones, only makes what I mention above, that much worse. Make sense?

 

[someguy2800]

If your cases are all the same volume then identical charge weights will fill the case about the same each time once the powder is settled. The problem with filling by volume is that powder does not fill a case consistently due to its shape. It's like weighing 5 gallon buckets of gravel, it will never be exactly the same. Most ball powders are not all the same partical size so the air space between particals is not consistent each time the powder is dropped in a case. Stick powder particals tend to bunch up on each other like throwing logs on a pile. With both ball and stick powders if you drop the powder in the case and note the height of the powder it's not always consistent. If you tap it on the table and shake it a bit to settle the powder you will get a different volume then you started with.

Let's say you have two cases and one holds 40 grains of powder and the other holds 42, which is a 5% difference in volume. You fill them both to the neck junction after settling the powder fully in the case. When the round first goes off the pressure for the first inch or so should be about the same between the two since they had the same loading density. Once the bullet is 10 inches down the barrel though the combined volume of the case and 10 inches of barrel makes the 2 grain case volume insignificant since the volume difference is a fraction of a percent. However because the smaller case has 5% less mass of hot gas filling the same volume, the pressure and velocity will be way down on the underloaded case.

Make sense? I personally like to fill a case at least 80% up to a slightly compressed load.

 

[Berger.Fan222]

I agree with the theory and always aimed for 95% or more for match rifle loads.

I've had some light loads with far less than 95% shoot really, really well over the years, through. Sometimes there is just a really sweet spot with powders that are not so sensitive to load density.

 

[Bart B.]

Some powders produce more uniform pressure curves (and velocity) with less load density than others with more density. Best example was tests for a new bullet a few of us worked up loads for. A ball powder had highest load density, lowest velocity and pressure spreads, yet worst accuracy at long range. Another powder had less load density, 3/10ths grain charge spread but best accuracy.

SAAMI has a procedure that helps test data for a load be more repeatable; it's called the SAAMI Twist.

A. Cartridges to be tested should be placed in a vertical position with primer-end down in a recessed holding block.

B. When the appropriate test barrel has been properly serviced and the chronograph reset, a cartridge should be lifted vertically from the block. It should be rotated slowly, end over end, in a vertical plane through 360° pausing momentarily when the powder is at the bullet end and again when the powder is at the primer end.

C. The cartridge is then rotated slowly, a minimum amount to enter the chamber, keeping the primer end in the lowest possible position until inserted gently and carefully into the chamber.

D. The cartridge should be seated in the chamber as far as practicable with the fingers. The bolt or breech mechanism should be closed gently in order not to disturb the position of the powder in the cartridge case. The object of this method of handling cartridges is to position the propellant powder at the primer end of the cartridge case by permitting it to fall gently against the primer while rotating the case.

E. The rate of fire should not be rapid enough to cause excessive heating of the barrel. The time between rounds depends on the equipment, as the barrel may be cooled by a constant stream of air on the outside or by directing air through the bore after each ten rounds.

F. Ammunition conditioning should be between 60° - 80°F (15.6° - 26.7°C).

G. A minimum of one and up to three warming shots should be fired before firing each series for record. The velocity and/or pressure of these shots may be recorded, but should not be included in the record of the sample.

 

[rfwobbly]

Agreed. This may be most apparent when using Tite Group in 38 Spcl.

In my experience, what you want is an amount of powder that covers the primer hole inside the case at normal barrel angles. Once the powder is ignited, it will generally burn consistently inside the case. So yes, you do need something like a minimum 70% case fill to fulfill this.

 

[MrBorland]

Some powders produce more uniform pressure curves (and velocity) with less load density than others with more density.

A friend of mine went to a talk by Bryan Litz, who (according to my bud) espoused relatively fast powders (regardless of load density) for max accuracy, the idea being that the faster the powder, the more uniform the pressure curve.

 

[Bart B.]

My observations of powders used for best uniformity in velocity, pressure, precision and accuracy are those about 3/4ths up the range from fastest to slowest suitable for a given cartridge and bullet weight. While the slowest ones do give higher average velocities, velocity spread is greater and accuracy/precision drops.

I'm convinced this is best proved by the benchrest folks throwing powder charges from a measure into the case and its got a 2/10ths grain spread but shoots smallest groups on average winning and record setting. Sierra throws such charges in reloads testing their stuff for accuracy getting 1/4 MOA groups with their match bullets at 200 yards.

There are bigger fish to fry than exact charge weights and load density trying to have all bullets' trajectories exactly the same.