Originally Posted by yulzari
The risk in using small arms powder in artillery is that the smaller grains cause a more rapid burn which gives a higher pressure and the cannon barrel may fail. If it does hold together (for a while at least) then the cannon ball will be projected faster rather than slower so it does not match the reported behaviour of Mexican artillery.
BTW the powder cannot be transported in it's constituents for mixing when needed. That would give you, at best, very bad firework powder. The incorporation of them needs much time and pressure. To get past transport shaking the constituents parts apart corning was developed generations before this war. The powder being pressed under high pressure into solid cakes which are then broken up and sorted through sieves into solid grains which will not break up if well made. It burns better than the earlier powdery 'meal' as the flash can pass through the interstices between the grains. I will spare you issues about adiabatic heating etc. By this time meal is only made for fireworks, rockets and some blasting purposes. Never for firearms but it would match the observed performances of Mexican powder.
I conclude it was probably made as badly made powder, badly corned, badly packaged, roughly transported and badly stored. Based upon low quality firework powder and deteriorated since making. Soft corning and poor incorporation will result in friable grains which will crumble in transport and be more hygroscopic in storage. I do doubt that even the most venal buyer and military would expect to get away with buying actual meal firework powder.