For perc guns, I'm in the capped-with-hammer-down camp. I always use a WELL FITTING nipple and cap combo, and make sure to lower the hammer on the cap and gently press forward to make sure its seated/conformed to the contour of the hammer cup. I want
NO GAPS between the hammer cup, cap, priming compound or the tip of the nip. IF the gun I'm toting even has a half cock, that's where it stays once I'm settled into my hunting spot.
Primed flintlocks stay at half cock with the stall on until fired.
A LOT of antique perc guns were built without even having a half cock position. I guess it really
was dangerous, cause all of those guys are dead now!!!
If the half cock notch of a capped gun fails during a fall, it's very likely that the cap is going to go off, due to the longer amount of free travel and the built up momentum.
Much less likely to happen if the hammer is flat down on the cap. It takes a
CONSIDERABLY SHARP whack to set off a perc cap when the hammer is already let down on it.
@Relic shooter has the right idea with the rubber washer to absorb any shock.
If you are curious about it, do some experimental testing yourself with an unloaded perc gun and a non marring dowel to blip the back of the hammer, being sure to mind the spur.