ORIGINAL: stubblejumper

How are you igniting that powder?With a match?How much powder at a time?
The powder in a cartridge case is held in a narrow column just in from of a primer that releases a great deal of energy that blasts a flame into that narrow column of powder igniting much of it at once.Therefore you get very complete ignition,and a very fast burn time.

That is not at all the case if you throw a pile of powder on the ground and throw a match into the pile.A match is not even close to as hot as a primer,and it is not driving a flame into much of the powder at once.
Try an experiment if you want to see just how the method of ignition effects the burn time.

Place 100gr of powder onto the ground and spread it as thin as you can.Throw in a match and see how long it takes to burn.

Take another 100gr of the same powder,and pile it into as small of a pile as you can.Light an oxy-acetylene torch with a large tip and quickly hold the torch flame right over the powder.Notice how much quicker it burns when it is concentrated together and has a much more powerful ignition source that can ignite much more of it at once.

When ignited in an unconfined state, smokeless powder burns inefficiently with an orange colored flame. It produces a considerable amount of light brown noxious smelling smoke. It leaves a residue of ash and partially burned powder. THE FLAME IS HOT ENOUGH TO CAUSE SEVERE BURNS.
The opposite is true when it burns under pressure as in a cartridge fired in a gun. Then it produces very little smoke, a small glow, and leaves very little or no residue. The burning rate of smokeless powder increases with increased pressure.