Thanks, all. The John Manton is advertised, way back then, as a self primer. When the powder is tamped down, a bit is supposed to leak out into the pan. My 16 ga. gun was loaded with 3fg as it seemed fine enough to self prime. The first shot detonated properly without added priming. The other barrel did not detonate. I primed the unsuccessful barrel, but still no detonation. I discovered that the unsuccessful barrel's flint was a bit loose. After repositioning and tightening the flint, it fired properly. Lots of slow, delicate preparation in operating a flintlock. No wonder that the percussion gun, and then the pinfire became more popular.
