According to some sources on the French army, the cuirass of the heavy cavalry was specifically 'proofed' by taking three musket balls at 'point blank' range, which I gather didn't have the same meaning as today (for example, 'point blank' for a cannon was something like 300 yards). Due to the difficulty of manufacturing such armor in bulk, this requirement seems to have been relaxed to one shot at 100 yards. We can perhaps conclude that bulletproof armor against the firearms of the time was possible, but just not very feasible outside of a few elite units that needed and could afford the protection. In any case, heavier armor would just mean a return to more powerful muskets and rifles, an 'arms race' that the armor would probably lose eventually.