Thanks, Randy.  The first time I read The Bullet's Flight, my small bear brain often found the technical aspects tough going, and now I'm much older and even less smart.  Still, maybe I'll give it another go.

Bill Lawrence

RSW, I have a copy of Mann's book with Pope's annotations.  He seemed to disagree that bore diameter pure lead bullets were the most accurate.  Several months ago I posted a scan of that page.   His writing is super hard to read though.

With respect to the finning problem at the base, Dan Theodore and I had a conversation about it years ago.  He'd done some testing and, with black powder, tested several wad combinations.  No wads and veg wads left finned bases on recovered bullets.  LDPE wads reduced the finning, and HDPE wads eliminated it.   He was of the opinion that pushing a bullet through the bore produced very different results on the base than what you see on a recovered bullet that was fired through the rifle. 

As an aside, none of my recovered bore diameter PP bullets have ever showed fins.

Chris.

I bevel my bases it leaves a place for the fins to go / eliminates them. The below target was shot with beveled bases that came out of a Hoch nose pour mould. In other words I remove the sharp edges on purpose. 

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JL and Dave
I respect your shooting abilities and you both know far more than I about precision BR shooting with cast lead bullets. BUT not to disparage what you guys have achieved, you shoot with smokeless powder. Dr. Mann's cast bullet experiments were fired largely with duplex loads. That propellant is more disruptive to a bullet's base on firing than smokeless. As in so many of his tests, bore diameter soft lead bullets upset completely to groove diameter. Not only did they completely fill to groove diameter, nearly all recovered bullets had bases that were oblique (not square) to the bullet body. So . . . not only did powder ignition mash the bases enough to fill the barrel, it made what were perfectly square bases oblique to the bullet body nearly every time.
Please don't mistake what I'm relating about Dr. Mann's experiments as acceptance or supportive of all his results. I started out re-reading his book to see what of his findings might still be relevant to today's shooters using black powder and/or duplex loads. I think his hang up on bore diameter soft lead bullets was kind of nutty but he was not looking only for tight groups. He was looking to eliminate the cause(s) of inaccuracy.

A sprue plate hold down spring as shown below is also very helpful in achieving flat bases using a base pour mould. I bought mine from LBT Bullet Moulds and the only place that I could find them. 

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Not much we can do about the displacement of metal at the base of the bullet, short of muzzle loading; but, what about the effects of nose slump? Does the large meplat of a nose pour mould offset some of that?

Correct me if I'm wrong but wasn't it Pope who suggested that by muzzle loading the bullet, any metal that was displaced by the barrels rifling was moved to the front of the bullet where it would have a lesser effect on accuracy than on the base.
Mike.