The British traditionally called bronze cannons "brass" to distinguish from iron pieces. There was no doubt in any military minds of the period that the material used was gunmetal or 90/10 (approximately) bronze. This was picked up in America where the old ordnance ledgers in the National Archives divide the ordnance into sections for "iron" and "brass." So it was definitely a semantics or tradition thing, not to be confused with the brass that is an alloy of zinc and copper, and which was never, to my knowledge, used for military cannon barrels.
This only changed in 1841, when the new system of artillery was introduced into the US Army. From then on, the bronze guns were always called "bronze" in the ledgers. The ledgers I'm talking about were compilations of the "ordnance returns" received from all ordnance depots, installations, forts, arsenals, etc. on a quarterly basis. These huge ledgers (some are like 3x4 feet and weigh 40 lbs or so) are in the National Archives, Washington DC, and are fairly continuous from about 1813 through at least the 1870's, with some gaps.