A friend had an early .50 Kentucky flint rifle - from back when the bores were actually .510". After he passed away, I bought it from his widow, along with his bag, horn, and measure. I mostly just use his load of 60gr FFFg, but I use .490", .495", and .500" balls in varying combinations of patches. The small lock is a bit fussy but very reliable when I pay attention, and I eventually had the patent-breech chamber opened up to .30" to simplify loading. I'm not especially good with open sights, so I'm happy with 1"-2" groups at 50yds semi-supported off a bench.
More recently, I picked up a used .32 Frontier flint carbine. The big lock on that little rifle still seems a trifle peculiar at times, but it is reliable. The little ~.20" patent-breech chamber requires care on loading. I'm still working on loads, but it is accurate.
Regards,
Joel