If you have a .40, then get a .32. It will still load with a 5/16" rod. That is the same size (tapered from .310" to about .27") I use in my .36.
2-finger loading with .022" patch and .350" ball. When the rifle had a .32bl. on it, I used the same rod and either .311" & .320" balls with the
same patches. The little balls conform into the rifling very easily. Using mink oil in the .32, the 50th shot loaded easier than the 1st one did in
the dry bore. No wiping. I was shooting 35gr. 3F GOEX, but if only shooting to 25yards, 20gr. shot just as well. The heavier charge was needed
for group shooting at 50yards. I use the same load in my .36 bl. and it does even better. The .32 had very narrow grooves & wide lands. I did
not have great faith in it so re-barreled the rifle.
This is the .32's muzzle crown. Some 320 grit wet/dry, rotated in the muzzle did the work from the machine cut by the factory.