I load cast in my 404 but never got good results with the 400 grainers plain base I cast and tried with various powders. Would not key hole but just would not group at all except with a low charge of slow powder which gave the odd hangfire unless packed with filler. Got hold of some 350 grainers with gas checks from another 404 owner and these performed superbly with AR2209. Turned a shoulder on the base of my own cast 400 grainers and fitted gas checks and yippee similar fantastic results using the same load. Without gas checks I noticed some lead splattering on the neck of the case.
I believe you will have much better luck with gas checked cast bullets in the big bores.