I find it interesting that absolutely no one is suggesting that your powder has gone bad. Almost all the reloaders out there believe that gun powder has an infinite storage life.
Guess what, it does not.
And you stored your ammunition in a shed. Heat is very bad for gunpowder.
Half of all the surplus IMR 4895 I purchased went bad.
The first 16 lbs, I used up eight pounds quickly. For whatever reason, I pulled the bullets on some of that stuff and found green corrosion on the bases of the bullets.
The last eight pounds, it sat around. When I opened the bottle top, it smelled bitter. Red dust flew around.
I gave it to a machine gunner guy. He put it in the laundry room. The bottle got a piece of laundry over it and over night acid gas from the bottle ate holes in the laundry. The guy then poured the stuff out over the lawn.
Since then I have had surplus 4895 powder go bad in the case. Green corrosion on the bottom of the bullets and cracked case necks.
That lot of gunpowder did not have a bitter smell. First I noticed I had a problem was cracked case necks when I shot the stuff. Then case necks cracked in loaded ammunition.
Eventually I pulled the bullet from 700-800 rounds, deprimed the cases, and dumped the cases. I used the primers in blasting ammunition.
I was able to talk to a Navy Energics specialist. He explained that powder deteriorates from the day it leaves the factory. The Nitrocellulose and nitro glycerin want to combine to form a lower energy molecule. Nitric gas is released in the chemical reaction. The rate of combination is directly related to temperature. The higher the temperature the faster the reaction. Powder contains stabilizers. The Navy samples its powders and propellants. If the powder is outgassing nitric gas (as determined by a paper that changes color (Methly Violet test, or Talliani test)), the stuff is tested to see how much stabilizer is left. If the amount is less than or equal to 20%, the lot is scrapped.
The Army does it different. The Army scraps small arms powders by time. Double based powders and ammunition are scrapped at 20 years, single based 45 years.
The military does not talk about this, but bunkers and ammunition storage areas have gone Kaboom due to old powder. That nitric acid builds up, creates heat, and the stuff blows up. It blows up inside the case or the shell.
http://www.liveleak.com/view?i=13c_1205681217
Before the powder goes boom!, that nitric acid gas is attacking your brass. It will cause corrosion on the bottom of bullets, and it will cause stress cracking first on the thinnest, most highly stressed sections of the case. Which happens to be the case necks.
The expert suggested that it is likely that surplus military powders are not on the market anymore due to liability issues. The stuff was scrapped because the military decided it was not safe to keep around anymore.
If the powder has turned red, or smells like acid, it is way beyond its safe limits.
I talked to Alliant powders. They told me that if the plating inside the old cans is has rust spots, the powder is doing that, and the powder should be dumped.
This is worth a look.
http://www.almc.army.mil/alog/issues...t_stab_eq.html