Since you started this thread, I dug out my copy of "Texas Illiad" again. In it, Hardin mentions several times that the Mexican black powder was horrible. Very undependable, and slow burning, resulting in very low velocity. After the battle of Concepcion, the Texans salvaged the lead but dumped the powder out of the cartridges.
Where was Santa Anna getting his powder? Was it supplied by England as well as most of the weapons?
7mm

"Preserving the Constitution, fighting off the nibblers and chippers, even nibblers and chippers with good intentions, was once regarded by conservatives as the first duty of the citizen. It still is." � Wesley Pruden