Heres a question for you black powder experts:  I have a  couple of Brooks moulds .40 cal.  one, a Postal style 330grs. drops at .4095 dia.  The other a Creedmoor @400grs. drops at .408.  I had these made up for a .40 G.M. brl. with .408 groove dia...for use with black only.  If i used these in a .410 groove brl., using Swiss 1 1/2..I know the black will slug them up to full groove dia...but what about accuracy?
   Paul Mathews wrote about trying bullets groove to .001" or so under groove dia.  Can someone shed some light on this?  Thanks!

I think it's best to use the full diameter if possible.  Mostly to ensure that the bullet is well aligned.  I have shot bullets that were 0.001 smaller than groove out of my Shiloh and they worked well, however that rifle has a very short 0.050" freebore.   

You are correct about bump up.  I shoot a lot of bore diameter paper patched bullets and they bump up quite well, even at 16:1.  They work because alignment is ensured by the fact that most of the bullet is out of the case and supported by the lands.

Chris.

Black powder fixed, I get best accuracy with groove diameter bullets. When muzzle loading, get best accuracy with bullets between bore diameter and groove diameter. 
The lands keep bullets in alignment. 

If seating oversize bullets in case necks, bullet diameter will be reduced. If you get lucky, bullet diameter will be very close to groove diameter. I prefer to use as cast bullets at groove diameter that are a close fit in the case necks. No sizing done to the brass. No muss no fuss and hopefully no miss. 

Another may get excellent results loading an entirely different way.  Or, your mileage may vary. 

         Joe. 

Try it. One of my most accurate loads uses a Brooks .456 cast 30/1 and slip fitted. It has shot good in every 45 I shot it in.

Conventuals wisdom is groove dia. To +.001 but Pope made his bases large .327-.328 so opinions vary.

40 Rod