I obviously did not do adequate research on what I purchased. I started cleaning my 66/74 French Gras in anticipation of shooting it. Slugged the barrel, and as the lead slug got close to the chamber, resistance increased quite a bit. Measuring with digital calipers showed .439" diameter. I ran another slug down the barrel, but not to the restriction, and came up with .456". I saw what appeared to be a sleeve at the point where the chamber throat reaches the rifling, as if someone drove in an insert to fire a smaller caliber bullet. Turns out the conversion from the Chassepot to the Gras involved driving in a full length insert, and that's what I'm seeing. I don't understand how the jump from .439" to a larger groove does anything positive.
I have 2 older Lee molds: the 445/290 and 445/288 hollow base singles. Would those of you experienced in handloading for the 66/74 consider those molds adequate for handling the jump in diameter? I intend to cast with pure lead, perhaps 40:1. What black powder charge is adequate for accurate plinking out to 100 yards? I have Pyrodex RS and Black MZ FFG.