Just picked this up and it seems SNK coded there own emu for 1,2,X,3. Does anyone know if 4 and 5 are emulated, or are they ports?. I have 4 and 5 collection but thougt they were ports, and my Jap Mslug 3 seems like a port (who knows) I cant imagine them coding a Atomiswave emu for 6 though.

Metal Slug 6 is the only one done differently. For the PSP, it has its own EBOOT and on the PS2 its own *.ELF.

The others' (MS1-5) files are all mixed together in *.POD archives for sound, sprites etc. so I'd imagine they are all emulated the same way. They were all on the same system anyway, so if you're gonna emulate one, why not just do them all?