DavidJust checked the 1847 Infantry Manual - and its the same as the later ones; load from cartridge followed by 'prime' (in this case with a cap) - "... half **** the piece (move the forefinger to throw-off the old cap), than take a cap from the pocket...". It's a well disciplined set of commands and motions; there's no explantion that I have seen suggesting mitigation of flash fires.
David
for your records I have three PH, a 1853 No 5233 / PH1858 No4938
PH !861 No1554.
Regards.
Pete.