Semi, have you slugged the bore of that rifle to see what the land to land actuallly is? Sounds like the .451s are simply too small. Check the barrel's crown as well and be sure to wipe extra lube off after loading. I think a hardness test on that lead might be in order.
Yeah,
UC. The bore is definitely .451. And the lead is dead soft.
The funny thing is that bronko had the same experience with these bullets as I did - the 420's sized to .451 fit the bore well, but the 340's sized in the same sizer were loose in the bore. The only difference I can figure is that I sized the 340's in one step from the cast diameter of .457 to .451 with olive oil as a lube. I could not do that with the 420's. I had to size them to .452 first. Then they wouldn't go through the .451 sizer with the olive oil, so I switched to Lee's case sizing lubricant for that step. Now get this, whenI measure both bullets around the base driving band with dial calipers, I consistently get .451 on both. [:-] Anyway, sizing them to .452 might do the job. I'd sure like to get these things to shoot. Have lots of lead. Also need to try felt wads instead of cornmeal. Just didn't have any .45wads on hand.