Compressed Powder Loads

One consideration of the actual powder charge is how it fits into the case. This means that the powder charge fills the case beyond the point where the base of the bullet will be when it is seated to it's proper depth. Naturally, the powder compression will vary depending on the seating depth of your bullets. The degree of variation you experience could range from no compression to excessive compression.

Compression of the powder will depend not only on the weight of the powder charge but on the size of the cartridge case. For example a 47 grain powder charge may be a lot to get into the case if you are loading a 243 Winchester. A charge of 65 grains might be questionable in a 7mm Remington Magnum, while it would be no special trick to get 100 grain charge loaded into a 416 Rigby case.

Heavily compressed powder charges (those that are fill more than 110% of the available case volume with the used bullet seating depth) are to be avoided. The results of excessively compressed charges might include cartridge overall lengths that continue to grow after the loaded round is taken from the bullet seating die. This is caused by the elastic nature of the compressed charge trying to return to its original volume. This means when the internal upward force of the compressed charge exceeds the strength of the case neck to grip the bullet the powder will slowly force the bullet out of the case. This can lead to jammed firearms and erratic ballistics, as well as very poor accuracy. None of these are desirable attributes.

Obviously, the reloader must be able to differentiate between acceptable light compression and excessively heavy compression of the powder charge. This is not a difficult evaluation.

The depth to which a bullet is seated will vary considerably with bullet weight, chamber dimensions and reloader preference. In a .338 Winchester Magnum, the base of a heavy bullet (i.e. 250 grains) may well need to be pushed past the shoulder / neck junction and into the case body. On the other hand, a light bullet (200 grain) bullet being seated into the same .338 case may need to be seated so that it occupies only the neck area.

It is not necessary to know the actual percent of powder compression to come up with the answer for this question: "Is this load's powder charge compressed acceptably or excessively?"

To determine the degree of compression, seat the selected bullet into an empty case to the desired overall cartridge length. Carefully secure the seating screw to insure a uniform seating depth on succeeding rounds. Also be certain that the die body is locked in a secure position.

Typical round to round overall length variation might range from +/- 0.003" to +/- 0.010" in the case of hollow point bullets. The variation is caused by the differences in individual bullet tips, the reloaders ability to measure repeatable lengths, tooling, and reloader techniques. You will, however, be able to establish an overall length for your components and tooling being employed by simply measuring a single cartridge. Make note of this overall length. Assume a potential variation of +/- 0.008" as the total range to be encountered. Experience has shown this to be a realistic approach.
Then place the desired powder charge, for which you wish to determine of the degree of propellant compression, into a primed case. Let the powder charge pour into the case normally. No attempt should be made to tap the powder charge into a minimum volume. With the previously adjusted seating die, seat a bullet into the powder charged case. Without delay measure and record the overall length of this round. If the charged cartridge's overall length is longer than the empty sample, plus the tolerance of +/- 0.008", the loaded round has an excessively compressed powder charge.

If the loaded length is the same as the empty sample (again with the tolerance of +/- 0.008"), re-measure after waiting 24 hours. If it still remains within the anticipated overall length tolerance, the load is probably compress within an acceptable range.

Final verification of acceptable compression requires assembling two or three boxes of ammo, checking each round for overall length immediately after bullet seating and again after 24 hours. If none of these loaded rounds exceeds the range earlier suggested (empty case length), the degree of propellant compression is fully acceptable.

The amount of force applied to the bullets base by compressed powder charges will vary. The method used when pouring the charge into the case and the propellant granulation size will both determine the extent of this force. The method described above allows a judgment as to whether or not the degree of compression is acceptable for a specific set of components and loading dimensions.

Note: Do not use any bullet crimp when using this method to judge propellant compression acceptability. Also, keep in mind that a neck sized case will produce less powder compression than a full length sized case. This is because the full length sized case will have a smaller internal volume.
If a heavily compressed load is acceptable, crimping the case to the bullet may provide sufficient bullet retention to prevent the problems that could arise with creeping bullets. Never use any compressed charge beyond the reloading data's maximum load recommendations. A compressed charge should never be used unless the bullet is seated to the specified overall cartridge length listed in your reloading manual data table.

Sometimes a heavy powder charge can be made to fit simply by switching the brand of cases. Even a change in lot number of the same brand can sometimes show a change in case volume due to individual lot variations in case wall thickness.

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