I have been having a blast running all kinds of tests this summer to achieve consistent Hi-Vel PPCB. I started out getting some decent groups at a 100 yds and then they went down the tubes. Then I was consistently getting "minute-of-berm-angle". I went through all kinds of lubes (dry teflon, moly, etc.) and finally went to an old NRA tip; dacron on top of powder, 2-3 grains of lithium grease on the dacron, and a lithium greased PPCB. That got me down to about 6" at a 100 yds. I tried all kinds of paper and bullet sizes with no particular improvement. What I did detect was severe powder fouling that seemed to open groups. It seemed like the PP wasn't making it to the muzzle. In the beginning, when I'd look at the muzzle from the side you could initially see the leading. After the change to lithium, the muzzle leading was gone, but I could still feel the powder residue buildup. That led me to brush after each shot..
For my .308. I first tried 45 grs. of H4895 under an LBT 150gr. PPCB and 41 gr.. H4895 under a Lyman 175 gr. 311291 . With 45 gr. H4895, I couldn't get much lithium grease under the bullet, so now I use 41 gr. H4895.
I'm still in search of the "Holy Grail" of match accuracy with hi-vel PPCB. I have not been able to replicate the NRA's success with hi-vel PPCB. Eventually, after making every mistake possible, I should get there. Hope springs eternal!
Best regards,
CJR