If ya keep'em cool and dry, Black Powder cartridges are still new enough technology that shelf life still hasn't been determened. It's my understanding that old originals still fire. The only problem would be if ya let lube or moisture contaminate them, same as with that faddish stuff, you know, the stuff that isn't suppose ta smoke!
When I first started CAS, I was using a 44 mag in my rifle and one pistol. Last year, just before the May members only shoot, I loaded up about 150 rounds of bp with a lube cookie and no card or wad between the powder. I was unable to go, and I later swithced over to .45 Colt, so those rounds have been sitting around in my shop for a year through the heat of summer and the cold of winter till last week. Having just purchased a used Marlin in .44 mag, I decided to have a little fun and experiment. I launched over 50 of them with only two missfires. One due to no powder, the other actually launched the bullet out of the barrel and hit the target, but the low recoil and noised led me to believe that the powder had been contaminated. So, if ya follow Butler's advice and keep 'em about room temperature and avoid moisture, they should keep well. Using a card between the lube and powder couldn't hurt, but may not be neccesary if you pan lube the bullets.
In 2001, I fired some UMC 45LC rounds loaded in 1892. Old balloon heads with hard , corrosive primers. All 43 I had fired.
I had them stored for twenty five years in a So. Cal garage. Don't know about the other 80 years. I'd say they last near indefinately.
My experience is similar to Rusty Gunn's. I acquired some old .44 W.C.F. black powder rounds with the W.R.A. Co. headstamp and small primers.
The primers were the old mercuric type which, from what I have read, were only good for 20-30 years or so.
They did not fire when struck with the firing pin, so I put the cartridges in a lathe and drilled out the old primers, leaving the rest of the shell intact. I replaced the primers with 1 1/2 Remington small pistol primers and the cartridges then fired perfectly fine in my 1873 Winchester.
Interestingly, after probably over 100 years of storage, the black powder did not loose any of its stength since the rounds clocked in the 1,200 f.p.s. bracket which was the factory advertised velocity for this cartridge.
I did dissect a few rounds and they contained 40 grains of b.p. and a 200 gr. lead bullet. The bullet lube was a greenish color and was pretty well dried out. The bases of the bullets were oxidized and erroded somewhat from being in contact with b.p. for so many years. Even so, 5 rounds went into 2" at 25 yards.
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