Thank's, Tony, I expected your reply, but I have found this Eley and Nobel information allready and going to stick to 520 gr bullet over 160-165 gr BP for barrels regulation.
Later I'm going to replace wood and to order stock from heavy dense wallnut with pistol grip (now it's straight stock) and to add needed 1 pound and probably I will try 650 gr bullet.
Have you ever heard about Blue Dot as BP substitute in large bores such as 577 and 600 BPE?

Geno.