Balance of springs, quality and angle of flint, Proper flint impact point for a given frizzen mass & geometry, proper metallurgy of the frizzen face all factor in a proper functioning lock.
Springs do need to be balanced and this thread and the other ones on broken flints and rebounding frizzens seem to all be evidence of a need fro better understanding of that balance
One thing that just occurred to me is that when looking at originals we have to factor in the different grades of arms that have survived.
When I was reenacting and in the museum business I had had the privilege of shooting a few originals in the past when I had much less experience. One thing I recall is that all of the 'besses and charlevilles had very heavy mainsprings and frizzens to match. Everything was overbuilt--big heavy everything. but it was just exactly what you would expect for a military musket that had to take a likk'n and keep on tikk'n. a well matched heavy spring set would help compensate for problems with a less than sharp flint or a wet and fouled frizzen. Then too in the military context frequent flint discards and changes were a matter of course.
The original trade guns were pretty much a match, though scaled down in size. I never shot any original strictly civilian long arms. but I'd hazard that they were not so overbuilt. I got to shoot a few pistols that belonged to a collector friend and the military pistols while smaller and lighter then the muskets of course still had very strong but balanced springs. Civilian pistols had much lighter, but perfectly balanced springs.
One of the reenactors I hung out with was as guy who was a member of the USA international ML team who shot on the musket team. The team muskets had both springs ground way down in terms of pounds and were much much easier to shoot accurately. you could practically flick the frizzen over with your fingertip. but they sparked perfectly every time. The springs were balanced to perfection, hammer necks had been adjusted for optimum angle, flints were carefully selected. Jim said that it was hard enough to shoot a musket accurately and all that heavy iron crashing around offline to the bore made follow through even more difficult. Just tuning the lock and lightening the springs, and keeping them properly balanced, made a world of difference in their team scores. It was all the difference between a recruits training rifle and a top military match or sniper rifle.
My point is that really heavy springs are not needed for civilian arms, though they might be for military ones, but that heavy springs or light; properly balanced locks will shoot reliable and that lighter ones might be able to be shot more accurately. I try to lighten springs on my few flintlocks the way Jim showed me, I've never had a broken off flint or a broken spring JMHO of course
