I thought I had read somewhere that Gamebore Regal was a re-brand of the discontinued Gamebore Pure Gold cartridges. If I remember correctly when I bought Gamebore Pure Gold a few years back, it had a pressure rating that was just under 8000 psi. I think it was around 7800 or 7900 psi at about 1200 fps (for 12 bore, 2 1/2 inch, 1 ounce loads), the paper case cartridges were less, maybe 6500 psi. But it looks like the Gamebore Regal is pushing 1425 fps now? So maybe they have changed the recipe?
Anyway, it seems that Diggory Hadoke endorses the Gamebore Regal cartridges for use in pre-war vintage English shotguns. So they must be okay.
Cartridges for Vintage Guns _____
TC
