Yeah, the combination rifling is really the worst of both worlds, but that's what you get when you have some people who are restricted to round balls (Pennsylvania) while others can shoot conicals, and finally some who want to shoot both.
The round ball only restriction is why T/C brought out the Pennsylvania Hunter, a flintlock with a 1 in 60" twist barrel specifically designed to stabilize round balls.
Worked great, they were accurate as can be.
When I was still living in Pennsylvania and hunting with my T/C I considered getting a replacement barrel from, I believe, Hastings, but the cost was prohibitive on my $14,500 yearly salary as a cub reporter, and then I moved to DC, so it became a moot point.
As for chert...
Chert and flint are actually in the same family, both being forms of quartz, and can be used in the same way. There are some differences, and not all flints and not all cherts are created equally. There can be significant differences in mechanical properties
Most English "flints" are chert, because England is LOADED with chert. Chert occurrs in nodules common in areas where there's lots of chalk, and if you know anything about England's geology, you'll know there's a LOT of chalk there.
I've been told bya couple of people that the best, bar none, English chert for flint making comes from a couple of square miles in Devonshire.