For Metal Slug 6 in MAME 0.139u1, you needed:
Metal Slug 6 MAME 0.139u1 (often used with MAME4droid ) is a unique experience because it marks the transition of the series from the Neo-Geo hardware to the more powerful Sammy Atomiswave The Game: A Fresh Chapter New Roster metal slug 6 mame 0139u1
Running this on via an Atomiswave dump (usually named mslug6.zip ) gives you the definitive arcade experience on a PC. For Metal Slug 6 in MAME 0