According to my copy of the 1954 edition of the "RULES REGULATIONS AND SCALES of the GUNMAKERS COMPANY and the GUARDIANS OF THE BIRMINGHAM PROOF HOUSE" the .577 3 inch nitro express case is a bit too small and the 600 nitro express is a bit too large. The dimensions of the .577 are rim: .750"(vs. 20bore min diameter rim of .766")and case diameter min: .662"(vs. .785" min dia. of 20bore.)
The .600 nitro express measurements are rim: .810" and case diameter: .700".
However, you can probably make either brass case work via swageing and expansion, whilst the .660 NE may be the best one to try to work with.
There may be some metric size cartridge cases that will work better, but I cannot think of any just now.
What is your opinion of the quality and condition of the VC barrel blanks that you received, as I am interested in procuring some myself from somewhere in the world.
Regards;
Bv
