I find that I almost never read about the use of Alliant Unique, without some side note about how "dirty-burning" it is, or how it "meters like cornflakes", or some other disparaging remark. In posts which include charge weight and projectile weight used in the caliber of interest, I note that frequently, the reloader is attempting to use Unique to develop very light loads. Such loads, by design, often do not develop chamber pressures which are much more than 60% of SAAMI/C.I.P. specs, and are well below the optimal pressure range for Unique to burn as intended. As a result, combustion is incomplete, leaving un-burned/semi-burned flakes of powder behind, and the propellant gets blamed, when it was in fact the reloader's mistake in propellant choice.
ANY propellant, when used in loads developing chamber pressures below what is optimal for it, will "burn dirty". This is true of Bullseye, 2400, W231, AA#5, 700-X, IMR-PB, and any other powder one cares to name. The fact that many fast-burning powders seem to "burn clean" at lower-power loads is not due to any inherent "cleanliness" of their formulation. The "clean burn" comes from the property of these propellants to achieve optimal chamber pressures with comparatively modest charge weights (I think the term is "brisance", but not certain).
Conversely, any propellant used in loads developing chamber pressures within the range that is optimal for its combustion will tend to burn far more cleanly. Many propellants seem to burn even more cleanly as chamber pressures rise above the lower limit of optimum. Caution is warranted, however, when approaching highest performance loads, as many powders tend to get "peaky" and inconsistent when pushed too hard.
The versatility of Unique is legendary, and I am a believer. I have used it to reload rounds from .32 ACP (a REAL p.i.t.a) to .44 Magnum and MANY .45 Colt rounds intended for use only in Ruger/T-C firearms. This versatility, however, depends upon the use of Unique to develop loads which generate TYPICAL operating chamber pressures for the round of interest. The assumption that Unique is versatile in the sense that it can be used to make loads ranging in performance "from mild to wild" in the cartridge of interest is often misplaced.
A few things can be done to assure that Unique burns as cleanly as possible:
1.) Increase the amount of crimp used on reloads. This will create resistance to the projectile moving forward, which SLIGHTLY increases the burn time, and can detectibly increase chamber pressure. When I'M adjusting crimp, I tend to do it in increments of 1/8th turn of the crimping die.
2.) Seat the projectile SLIGHTLY deeper. This works in ways similar to increasing crimp. Even very minor adjustments can yield big differences. In smaller capacity cases (9x19mm & smaller), I advise adjusting seating depth (if at all) in the smallest increments available to the shooter.This option may not be available to the reloader, if the rounds are used in a pistol that is picky about what overall length cartridges it likes to feed.
3.) Magnum Pistol Primers: In 40 years of reloading, I've NEVER had an instance in which Unique REQUIRED the use of magnum pistol primers for uniform ignition. In many cases, however, their use has made Unique burn more cleanly. I discovered this serendipitously when I ran out of standard pistol primers, halfway through a batch of ammunition. I consider this step a "last resort", but it does seem to help every time I've done it. As always, when switching to a different primer, reduce the charge weight and work up.
I've listed these steps in order of desirability and convenience and STRONGLY suggest trying them one at a time (i.e., DON'T try all 3 on one batch of ammo!). If step 3 is contemplated, I suggest returning to the longer overall cartridge length, before switching over.
Even if all 3 steps combined are used with Unique powder, it probably will never win the "cleanest burning propellant" award. However, if due attention is paid to steps 1 & 2, Unique should burn with acceptable cleanliness, and its fallacious reputation for being a particularly dirty-burning propellant will be seen to be largely undeserved.
METERING: Unique will win no contests in this regard, either, but again, I think the matter is over-stated. Years ago, in an excessively meticulous project, three of us shot .38 Special reloads consisting of 3.0-4.0/Unique/LSWC, from 3 different revolvers. Half of the rounds were loaded with powder charges thrown in an RCBS powder measure, the other half were loaded with charges individually weighed and trickled. At the 4.0/Unique level we could discern no measurable difference between "thrown" powder charges and individually weighed charges, with respect to group size. At 3.5/Unique, one of us thought he could see a measurable difference in group size "between thrown and weighed". At 3.0/Unique, two of us were certain that there was a measurable difference between the two batches.
From this, I've found that charges of Unique that are greater than 4.0 grains meter well enough that accuracy does not suffer as a result.
Additionally, the "flakiness" of Unique is one of the reasons I use it when reloading larger capacity pistol and revolver cartridges. Unless the charge weight is too low to begin with, it is almost impossible to double-charge most rounds with Unique. The same cannot be said of many of the cleaner-burning, meters-like-water, ball powders on the market. Yes, it is true that all responsible reloaders should visually check powder levels in cases before seating the projectile. I make it a point to do this, and others should, also. But it also helps to know that a double-charge will quickly become readily apparent.
Okay, end of sermon...
ANY propellant, when used in loads developing chamber pressures below what is optimal for it, will "burn dirty". This is true of Bullseye, 2400, W231, AA#5, 700-X, IMR-PB, and any other powder one cares to name. The fact that many fast-burning powders seem to "burn clean" at lower-power loads is not due to any inherent "cleanliness" of their formulation. The "clean burn" comes from the property of these propellants to achieve optimal chamber pressures with comparatively modest charge weights (I think the term is "brisance", but not certain).
Conversely, any propellant used in loads developing chamber pressures within the range that is optimal for its combustion will tend to burn far more cleanly. Many propellants seem to burn even more cleanly as chamber pressures rise above the lower limit of optimum. Caution is warranted, however, when approaching highest performance loads, as many powders tend to get "peaky" and inconsistent when pushed too hard.
The versatility of Unique is legendary, and I am a believer. I have used it to reload rounds from .32 ACP (a REAL p.i.t.a) to .44 Magnum and MANY .45 Colt rounds intended for use only in Ruger/T-C firearms. This versatility, however, depends upon the use of Unique to develop loads which generate TYPICAL operating chamber pressures for the round of interest. The assumption that Unique is versatile in the sense that it can be used to make loads ranging in performance "from mild to wild" in the cartridge of interest is often misplaced.
A few things can be done to assure that Unique burns as cleanly as possible:
1.) Increase the amount of crimp used on reloads. This will create resistance to the projectile moving forward, which SLIGHTLY increases the burn time, and can detectibly increase chamber pressure. When I'M adjusting crimp, I tend to do it in increments of 1/8th turn of the crimping die.
2.) Seat the projectile SLIGHTLY deeper. This works in ways similar to increasing crimp. Even very minor adjustments can yield big differences. In smaller capacity cases (9x19mm & smaller), I advise adjusting seating depth (if at all) in the smallest increments available to the shooter.This option may not be available to the reloader, if the rounds are used in a pistol that is picky about what overall length cartridges it likes to feed.
3.) Magnum Pistol Primers: In 40 years of reloading, I've NEVER had an instance in which Unique REQUIRED the use of magnum pistol primers for uniform ignition. In many cases, however, their use has made Unique burn more cleanly. I discovered this serendipitously when I ran out of standard pistol primers, halfway through a batch of ammunition. I consider this step a "last resort", but it does seem to help every time I've done it. As always, when switching to a different primer, reduce the charge weight and work up.
I've listed these steps in order of desirability and convenience and STRONGLY suggest trying them one at a time (i.e., DON'T try all 3 on one batch of ammo!). If step 3 is contemplated, I suggest returning to the longer overall cartridge length, before switching over.
Even if all 3 steps combined are used with Unique powder, it probably will never win the "cleanest burning propellant" award. However, if due attention is paid to steps 1 & 2, Unique should burn with acceptable cleanliness, and its fallacious reputation for being a particularly dirty-burning propellant will be seen to be largely undeserved.
METERING: Unique will win no contests in this regard, either, but again, I think the matter is over-stated. Years ago, in an excessively meticulous project, three of us shot .38 Special reloads consisting of 3.0-4.0/Unique/LSWC, from 3 different revolvers. Half of the rounds were loaded with powder charges thrown in an RCBS powder measure, the other half were loaded with charges individually weighed and trickled. At the 4.0/Unique level we could discern no measurable difference between "thrown" powder charges and individually weighed charges, with respect to group size. At 3.5/Unique, one of us thought he could see a measurable difference in group size "between thrown and weighed". At 3.0/Unique, two of us were certain that there was a measurable difference between the two batches.
From this, I've found that charges of Unique that are greater than 4.0 grains meter well enough that accuracy does not suffer as a result.
Additionally, the "flakiness" of Unique is one of the reasons I use it when reloading larger capacity pistol and revolver cartridges. Unless the charge weight is too low to begin with, it is almost impossible to double-charge most rounds with Unique. The same cannot be said of many of the cleaner-burning, meters-like-water, ball powders on the market. Yes, it is true that all responsible reloaders should visually check powder levels in cases before seating the projectile. I make it a point to do this, and others should, also. But it also helps to know that a double-charge will quickly become readily apparent.
Okay, end of sermon...
