This is true. I have done it. Was doing a demonstration at the local gun club and made a point of mentioning that the gun doesn't need to be overprimed and will in fact possibly fire without any prime. I attempted to fire.the rifle with an empty pan and it did indeed fire on the first try just as fast an ignition as had I primed the pan.One more thing new flintlock shooters need to know...A flintlock can and will go off with no powder in the pan but loaded with the main charge!!!
So, if you are shooting and don't get a spark, knap the flint but do not see if it will spark on the loaded gun or you may be cleaning out your pants...
Also, if you are going to pull a charge with powder behind it, pour water or oil down the barrel to neutralize the powder...
Have fun!!!