I had this problem on my Chambers Late Ketland lock. This lock has a powerful mainspring, and it throws sparks like a cutting torch. But the frizzen does not open very far, so that the heel, that is to say the edge where the frizzen meets the pan cover, is not very far away from the flint in the fired position. The frizzen spring does not have enough tension to stop the frizzen from bouncing back and breaking off the flint. So I did two things. First, I made a new rolled for the frizzen spring, so that the toe of the frizzen has farther to ride to get back over the roller, and I heated the spring red and spread her leaves a little, also to increase her tension against the toe of the frizzen. Of course, I polished, hardened and tempered the spring again. The result in this case anyway, was that I no longer get smashed flints.
The bottom line is as Trent has said: the mainspring and frizzen spring have to be balanced. There are locks that are perfect right from the manufacturer, and there are those that need tuning. It's all part of the fun. So enjoy the ride!