In all the years I worked on UnCivil War Period Rifle (Short or “2 Band” rifles) and Rifle Musket (3 Band) locks, I only saw a very few where hammer blowback was a problem. In all cases, the nipple/cone had been continued to be used long after the vent hole had opened up too much from wear. In addition to that, either the locks were of very cheap quality and had “light power” mainsprings or someone thinned the mainsprings in an attempt to lighten the trigger pull. The latter was very uncommon, though. I don’t believe I ever saw an original Springfield or Enfield Rifle or Rifle Musket lock or a real English made Parker Hale Arms lock do it because their springs were so robust and even when folks did not change the nipple when they should have. Of course, this is with powder charges and bullets that did not go well over normal pressures.
I should also mention that many original guns in use by NSSA shooters since the 60’s and 70’s have had FAR more rounds fired in them than most any original guns did during the UnCivil War. This is a testament to the original lock and parts designs being so robust and usually so well made that they stood the test of use so well over time.
Of course, just as in my modern career of being a Military Armorer, I have seen some very strange and unusual ways people have broken or disabled their locks and guns from lack of knowledge, accidents, neglect or even abuse – but one can’t hold the guns at fault for that.
Gus