I understand that the Mexican Army of 1846-1848 was armed with the British "Brown Bess" musket (probably the "East India" pattern, introduced in 1803, or just possibly some the 1839 pattern, the last flintlock model introduced, which was manufactured to better standards than previous models but otherwise no different).

The British Army had begun converting their existing stocks of muskets from flintlock to percussion firing in 1840. (A fire at the Woolwich arsenal in 1840 destroyed large numbers of muskets under conversion, so a new version with percussion firing was manufactured in 1842). However, I doubt whether the Mexican Army had received percussion weapons, or converted any of their existing stocks.

I once heard some folklore that because the powder used in Mexican ammunition was very old and of variable quality, Mexican troops were in the habit of pouring the powder from two cartridges into the barrel, followed by only one bullet. If the powder had not deteriorated, the resulting recoil threatened to dislocate shoulders and the large flash from the priming pan could give painful facial burns. As a result, Mexican soldiers habitually fired from the hip, or at least aimed only cursorily.