Download the cheat.zip file for MAME 0.139 from cheat databases. Place it in the root MAME folder and enable cheats via mame.ini (cheat 1). You can then:
Save states are less stable on Atomiswave games, but pressing F7 (save) and F8 (load) may work. metal slug 6 mame 0.139u1 rom
Crucially, Metal Slug 6 requires the Atomiswave BIOS file. Without it, the game will not boot. Find a file named awbios.zip (from the same era as 0.139u1) and place it in the roms folder alongside your mslug6.zip. Download the cheat
To verify you have the legitimate ROM for this version, compare checksums. For the main program ROM (315-5221a.bin), the correct CRC32 should be E2D6D982. The full set’s SHA1 should match community-maintained DAT files from Redump or No-Intro for MAME 0.139. If your ROM has different hashes, it may be for newer MAME versions and will fail. Save states are less stable on Atomiswave games,
The actual mslug6 ROM for MAME 0.139u1 was rare even in 2010. Most “complete” MAME sets at the time omitted Atomiswave games because they were considered non-working. Finding it required digging through:
The file size was around 80 MB – tiny by today’s standards, but huge compared to Neo Geo ROMs. It required the Atomiswave BIOS (awbios.zip) placed in the roms folder as well.