NitroX,
I had a mate who had a .505 Gibbs in the CZ 550. Both myself & my youngest lad had a go of this rifle & it shot rather well to say the least!
He got a sizing die made up from Lee to size the Woodleigh 440gn SP's down to .505", it was a fair bit of an effort to do so but we got there eventually sizing them down to .505-.506" average. Unfortunately he had an offer to good to refuse so he sold his rifle before we could get these into service.
You would need to size these in a very strong "O" press & make sure the sizer works on the bottom/top of the stroke so that mechanical advantage is greatest.
I found that Hornady case lube in this instance was better than the Lee Alox?
David Commons from CBE has 0.509" cast projectile(s) - guess sizing it down to 0.506" would not be too hard if you wanted cheaper again.
Regards
93x64mm