A-D designates how big a barrel is, this is for swamped (tapered and flared barrels). An A weight might be 3/4" at the breech, a D weight might be 1 1/8" at the breech.
An A weight barrel is slender with a .40 cal as big as you can get in this configuration, B weight would go to .54, C weight to .58 and D weight to for big bore stuff like 10 or 12 ga, or large rifled bores.
The style of gun you build will determine what lock is right for it, for instance, a Lancaster style gun will have a Germanic lock like a Siler most of the time. A southern rifle will have an English lock similar to a late Ketland most of the time.
Of course lock styles intermix on the originals but they do tend to go one way or the other most off the time.
