CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
WC844 is what H335 is, except WC844 is bulk grade and H335 is canister grade. Canister grade goes through extra processing steps to control the burn rate to ±3% which is good enough to make sure load manual data remains valid with different lots of it. Bulk grade has a wider (occasionally much wider) lot-to-lot burn rate tolerance, necessitating the adjustment of loads to accommodate whatever burn rate the particular lot actually has. Commercial manufacturers buy the bulk version because it's fewer processing steps make it cheaper, but the manufacturers have pressure test guns to enable them to adjust the charge weight to the particular lot. Hand loaders usually do not.
As suggested earlier, it may be surplus for any of a number of reasons. Worst case, it's surplus because it's burn rate turned out to be so far off the norm that it's not serviceable for 5.56 ball ammo, which was WC844's original purpose. Second worst case is that it's very old and can't be trusted to be loaded and not fired promptly. (Double base propellants age and break down faster than single-base stick powders do. The military only keeps them 20 years, where they keep single-base stick powders for 45 years. Both are conservative numbers under good conditions, but you have to keep an eye out for deterioration anyway.) Pull down powder is going to be from surplused out ammo that was probably at least 20 years old, though occasionally it's because its pressure got too high as it aged. However, at it has the advantage that at least you know it qualified for loading in 5.56 at some point in time. Finally, best case for fresher powder, is it was surplused out because it was overstocked, but is perfectly serviceable.
But there's no way to know which reason it was. At best, with the wider burn rate tolerance, you're in for some careful experimenting to ensure you can roll a serviceable load out of it. Board member Hummer70, who used to work for Aberdeen proving ground said that the worst case he ever saw was a lot of bulk powder 30% off the nominal burn rate and that didn't qualify for its intended use. The second worse was, I believe, 20% off. Burn rate and pressure do not track. In QuickLOAD a 30% high burn rate with WC844 can be compensated for by reducing charge weight about 19%. So if you have a known good maximum load with H335, multiply it by 0.81 to get a first test load with the surplus lot. The drawback is that if the burn rate is normal, this will be a very slow, dirty burning load. Worse, if it's burn rate is 30% low, that reduced load could leave a bullet stuck in the barrel.
I can think of a way around this predicament. First check that the powder has no sign of deterioration by sight or smell (reddish fumes or dust; acrid acid smell), but I hesitate to recommend to anyone without some experience handling chemicals. It involves working up a middling load with fresh H335 bought from the store, then replacing 5% of it with a sample of the surplus powder blended carefully in and firing them over a chronograph side by side, under the same conditions, to see if the blend takes velocity up or down. If it goes up, the surplus is faster. If it goes down, the surplus is slower. Then, at least you know which direction the charge weights for the surplus will have to go.
