Metal Slug 6

Metal Slug 6 is a run and gun video game for the Sammy Atomiswave arcade platform. It was released in 2006 and is the latest game in the Metal Slug series. There was a PS2 port in September 14, 2006. It is also featured in Metal Slug Anthology for the Wii, PlayStation 2 and PlayStation Portable. Metal Slug 6 was the first main game in the franchise not to run on SNK's Neo Geo MVS arcade hardware.

Story
The story listed on the Metal Slug Anniversary site is as follows:

A farewell to arms―it was all supposed to have finished with that final battle.

Morden's third attempt at world conquest was over. But his whereabouts still remain unknown.

But even after with the remnants of Morden's army neutralized, our heroes' requests for discharge from the service were still refused.

For a brief moment, there was peace. But the frequent chatter of late intercepted by the expanded and consolidated Intelligence Agency suggests this peace will not last long.

Recent intel includes testimonials from people who have seen UFOs, aliens, and giant creatures. Fortunately, there have been no reports of damage or injury, but once can help feeling the mars people, maybe even Morden, is behind all this. And finally intelligence officers acquired conclusive proof:

A group resembling Morden's army had his themselves in a mountainous region and was gathering weapons and building installations.

Yet there were still doubts: General Morden's was nowhere to be seen, the objectives of the group were unclear, and whether this group was in fact Morden's army remained inconclusive.

Although it was concluded that this could not be a military group by any standard, those at HQ, particular sensitive to anything concerning Morden, however, quickly summoned the usual four:

Marco and Tarma from PF Squad and Eri and Fio from S.P.A.R.R.O.W.S.

Only moments away from deployment, no one attempted to voice their opinions.

They stood face to face with two new but yet somehow familiar faces ready to join their team.

"It's not that we don't trust the four of you," the commander spoke, "but word from the top dictates that you'll be working with these two private contractors for this operation."

"Ralf Jones here. Let's get this baby done!"

"Clark Still. It will be my honor if I can be of assistance to all of you."

With their newly furnished weapons in hand, this new team began their parachute drop into hell. This is the last time!

Gameplay
Metal Slug 6 returns to the Rebel-Martian alliance featured in Metal Slugs 2, X, and 3, but on a much broader scale. Rather than repeating the previous games' events of the Martians breaking the alliance and the Rebels assisting the player in turn, the player now teams up with the Rebels and Martians to combat an even greater threat. However, the game roughly takes place before Metal Slug 4 and 5.

There are now two modes of play the player can choose from right at the beginning: Easy and Hard. Easy mode lowers the difficulty of the game and changes the player's default weapon to the Heavy Machine Gun (that, like the pistol it replaces, has infinite ammo); however, the game ends just before the fifth and final mission. Also, when you continue after dying, you get a bomb icon instead of the Heavy Machine Gun icon.

As with previous installments, Metal Slug 6 adds a number of new Slugs (including a cart-toting Donkey Slug resembling a zumbooruck, the subterranean Slug Digger, and a Slug Gunner Prototype) as well as a new weapon, the Zantetsu Sword [Z], which allows the player's melee weapon to emit lethal energy waves that can neutralize enemy firepower.

New techniques are now present as well, activated through certain button combinations. Characters can now throw away one gun power-up's worth of ammunition, to give to the other player or simply discard altogether. A secondary melee attack is also available. Conversely, the sliding technique from Metal Slug 5 has been removed.

Metal Slug 6 introduces a new play mechanic dubbed the Weapon Stock System. Two gun power-ups can now be carried at the same time. Players can switch between the two weapons, or simply put them both away in favor of the default weapon. When obtaining a new weapon power-up, it will automatically occupy the inactive slot (or, if both are holstered, replace the less recent weapon of the two.)

The score is now multiplied by powers of 2. The faster the speed at which enemies are killed, the higher the power, as a meter at the bottom of the screen shows. When it says "Max" enemies will drop coins for an extra high score.

Characters
The number of playable characters has expanded for the first time since Metal Slug 2; the original team of Marco Rossi, Tarma Roving, Eri Kasamoto and Fio Germi is joined by newcomers Ralf Jones and Clark Still of Ikari Warriors and The King of Fighters fame.

As a series first, each playable character now possesses his or her own unique attributes. The amount of ammunition received from weapons, defensive ability of vehicles, and running speed vary from person to person, each of whom possess a special ability:

When all 4 missions are completed on Easy mode the game says "To be continued", meaning that the game must be played on Hard mode to continue with the final mission.

New Slugs
The Metal Slug, Type-R Metal Slug, Slug Flyer and a Slug Gunner prototype make an appearance in the game, as well as some new ones.

Donkey Slug: A donkey with a mounted vulcan cannon. Like all animal slugs, the donkey provides no protection to the rider. Later on in the level, the Donkey Slug acquires a two-wheeled out-house cart mounted with a Metal Slug cannon that replaces the player's grenade with metal slug cannon shots.

Slug Digger: A multi-purpose subterranean digger as well as a helicopter. It has retractable drills on the left, right, and bottom sides, which are used to drill into the earth, and sports two vulcan cannons, which can also be used to drill through things by firing bullets into the dirt. The players cannot get out of the slug, neither can they perform the metal slug attack with it. If the slug digger is destroyed, the player is killed. Its cannon shot is a mine that stays in place until an enemy touches it or it is shot.

Slugloid Z: It is similar to the Slug Gunner from MS5, but it is armed with two vulcan cannons. The player is exposed to enemy attacks.

Trivia

 * This game, Metal Slug 7, Metal Slug XX and Metal Slug Defense are the only of the series to receive characters from the other franchises, whether from the company itself or from other companies.
 * This makes the game, along with Samurai Shodown (2019), Art of Fighting 2 and Fatal Fury Special the only non-crossover games to receive characters from the other franchises.