soft alloy bullets

[loophole]

I've just discovered this forum, but I've been shooting cast bullets for 30 years, mostly in handguns and 45-70's.  I've had best accuracy at black powder velocities, whether using smokless, black or duplex loads, with alloys that are fairly soft--about 4ww/1pb or 20-1.  I'm shooting them 1 to 3 thou over groove diameter, so obduration shouldn't be a factor.  this is ture of 45-70, as well as 44 mag at 44 sp velocities in my model 29 and in a browing lowall.  Harder alloys don't shoot as well.
I know we need hard alloys at higher velocities, but is there some converse rule that says we should use softer alloys at slow velocities?

[Veral]

No, there is no rule.  Here is what is happening for you.
 
  Because you aren't fitting the guns properly, soft alloys are mandatory so they can slug up to fit the guns, and get best accuracy.

  The negatives are.  Your best is inferior to what you could be getting with hard bullets properly fitted, and velocity is more limited, with any lube you use.  Using LBT lube will allow the highest possible velocity potential.

  It is likely that some will tell you otherwise for very light loads, but read carefully what I've written.  Bullets which are too hard to obturate at the pressure of interest, MUST fit before the primer pops.  Those who counsel other than my advise don't understand or do this.  

  All instructions for setting guns up properly are included in my Lap kit, and all slugging instructions are spelled out in mycatalog and in the instructions sent out with our push through and throat slugs.