you need a 416 Rigby bolt face ( case dia head is 14.82 mm in 338 LM LAPUA BRASS ) but any 550 mag can work , just need to enlarge the bolt face and modify the exractor that better because if the work is done by a real skill gunsmith by this way you are sure that your bolt face is SQUARE / YOUR BOLT AXIS .
I have never compare mag box of the CZ 550 I don't know if there is diffrence between 375 HH box and 416 Rigby box .
good shooting
DAN TEC