This word has a number of meanings according to the OED, but for me, "hammerstall" is the little leather sheath that fits over the hammer of a flintlockmusket. The hammer is what the flint strikes, when you fire the gun, to produce a spark that ignites the powder in the pan, which in turn ignites the powder down the muzzle, and the bullet is propelled... etc. (When a gun just makes the spark and does not fire, it's a "flash in the pan"--cool, huh?) As a safety precaution, the hammerstall stays on the hammer until you are ordered to "Make ready!" (you take off the hammerstall and full-cock) and "Present..." (you aim the musket). The hammerstall is attached to the triggerguard with a leather thong so it's always available, though I don't know if that was true in the eighteenth century.