|September 13, 1999, 09:59 AM||#1|
Join Date: March 19, 1999
I have a question concerning the powder charge associated with jacketed bullets vs. lead bullets. I am new to reloading (about 3000 rounds so far), and I am trying to understand something that I have observed in the loading data.
If cast lead bullets seal better against the bore than jacketed, or if they have a greater lubricity than jacketed bullets, or if there is some combination of similar mechanisms responsible for different charge weights; then there should be a consistent trend in the powder charge differences between otherwise equivalent lead and jacketed bullets. But this trend is not evident in the loading manuals. For example, consider the load data in Lyman's 47th manual for 230 gn jacketed round nose bullets and 225 gn lead round nose bullets in .45 acp.
Even if the 2.2 percent difference in weight between these two bullets is significant, it is a constant. There are 6 powders that are listed for both bullets. Here is the difference in charge weights for a couple of powders:
For 700X, the minimum charge weight is increased by 0.4 gn from jacketed to lead, and the maximum charge is 0.2 gn greater.
For 4756, the minimum charge weight for the lead bullets is 0.5 gn LESS than that for jacketed bullets, and the max is 0.5 gn MORE than for the jacketed bullets. ???
For Bullseye, the minimum charge for lead is 0.2 gn MORE than for jacketed, and the max is 0.3 gn LESS than for jacketed. ??????
The only explanation that makes sense to me is that, although there is a high degree of uncertainty in the pressure measurements, the measurements are performed once for each charge/bullet combination and the testing stops when the maximum pressure is obtained. This explanation is pure conjecture on my part. It could be that the measurements are highly repeatable, and that there are logical and deterministic phenomena that will explain the apparent inconsistencies.
If anybody can provide some insight on this matter, I would be grateful. Thank you.
|September 13, 1999, 12:06 PM||#2|
Join Date: November 19, 1998
I like max charges in my .357 Mag., and can report the following in 125-gr bullets.
WW296, Oregon Trail really hardcast lead bullets go 75-100 fps faster, even when I accidentally used standard primers instead of magnums.
With Alliant Power Pistol, the difference was 100-125 fps, and the loads showed pressure signs.
My WAG *theory* is that the pressure curve of these slower powders provides a longer-duration obturation "bump" on the lead slug than they do with jacketed bullets. The bullet is continuing to try to collapse backwards against the base, resulting in outward pressure against the bore (including a perpendicular thrust component) and friction keeps the pressure up. Result is higher velocity.
The effect can be different with other powders (burn pressure/time curves can be different even though the peak is the same), calibers (the curves can be different with different overall peak pressure levels)*, and bullet weights.
But overall, the lead bullet safe max charge weights are most commonly a bit lower than for jacketed bullets. I've seen _some_ identical max charge weights listed for lead/jacketed, but almost always with light bullets using heavy charges in high-pressure cartridges.
And don't forget the effects of longer and shorter bearing surfaces between different bullets, jacket thicknesses, and solid copper construction... This makes some "apples" look more like "oranges" compared to others of the same weight.
*--do NOT try to apply the lead/jacketed differences of a powder tested in a medium- or high-pressure round like .40 S&W, to a lower pressure round like .45 ACP or .38 Special. You may note that DIFFERENT powders give the higher velocity between the two cartridges, showing that generalizations regarding "slow" and "fast" powder performance can fall apart when comparing performance at different peak pressure levels!