I have experimented with bumping bullets some in the past here is what I have learned.
Richard Corbin will make dies for you that will conform to the dimensions you need he will make them for straight or tapered bullets.
You can bump a base band up maybe a thousandth or so but beyond that you are deforming the bullet too much to give good accuracy.
Fill the grooves with lube first. The best way is to pan lube and stick them in the oven at about 200 for 10 minutes or so this will insure that all the lube groves are full, any voids will result in a bad bullet coming out of the press.

40 Rod

Guys,  What I have is a former Swedish Rem RB  that came  in 8X58R chamber.  The chamber was  ruined by a weld.  I set the barrel back about an inch, bored out the chamber and added a chamber sleeve , that I chambered  .32-40 .  Had lots of advice along that  road.   So now I have it chambered .32-40  with a bore of .314"  and groove .326".  That has lead me to seek a .326-.327" Dia. mold,  thus the first question at the start of this thread.  It sounds like there are a few  "home" remedies such as Beagling,  and drop  bumping,  and  Swage bumping up.  Probably the real solution will be a custom mold , but It sure would be nice to be confident that the diameter chosen will be liked in the rifle.  So I will  try some of the home remedies as a start, and them maybe go to a custom mold when I am sure ( I hope) of the diameter  the rifle likes.  Thank you all for thoughts and advice.  WB

Water Bug,

Here's one of them:

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Any one want to cover lead alloy best for black powder?
I really doubt that any bullet much harder than what--10 BHN?--would bump up to seal the bore.
PS: if you swage the bullet, be sure it is already lubed.

If you're shooting bore dia bullets any lead alloy from Lyman #2 all the way down to say 30-1 will bump up just fine. Bullets cast with higher tin contents usually cast better, and will be slightly larger in dia, and fractionally lighter.
As for accuracy you just have to experiment, that's what makes it fun.   
Mike.

Water bug
20-1 is a very good place to start and you may never need to change that, perhaps for hunting a softer bullet would be better of course.
As for seeing me at the range it will be quite a trip for you.
Mike,
Canberra
Australia.