difference

Hi all, I am new to reloading. I have been taking my time doing a lot of reading and planning. Last weekend I did some testing of 4 different loads. All loads were identical except I used 3 different types of brass and the 4th load I tested for any differences from my case length being 1.750...

Can someone here explain to me in laymen's terms how black powder (classed as an explosive) differs from smokeless powder (classed as a propellent I believe) differ once they are confined to a specific space -- say a bullet/gun chamber? BP rapidly "flashes" when lit while it's unconfined...