I came across a can of black powder that i forgot I had. Its at least 20 years old. Is it ok to use?

As long as it is dry, yes. If it isn't it won't burn anyway. Black Powder will last virtually forever if properly stored. If not it gets weaker!

I currently using FFg and FFFg black powder that my dad bought in the mid '70s. It has been sealed in factory cans. Shoots just fine.

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John 3:16
WAR EAGLE!

If the can has been opened and not stored properly....for a extended period it could be questionable. If it goes bad it will have a bit of a sour smell.

But if well sealed and properly stored it will last a very long time.

Giz

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Non gratum anus rodentum

The OP was asking about BLACK POWDER! It doesn't go bad like smokeless can.

Thank you for setting me straight....

Obviously, I must know nothing about BLACK POWDER.....

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Non gratum anus rodentum

Clearly you don't. So the question is, what have you been loading all those cool flintlocks with? Bullseye?

UNIQUE...

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Non gratum anus rodentum

I have some that must be 30 years old, stored under decent conditions. It's fine. As a matter of fact, I loaded up some BP .45 Colts back around 1972 and they still make a very satisfying boom every time.