Overview
An upgraded version of Metal Slug 2, titled Metal Slug X, was released in March 1999 for the Neo Geo MVS. The game used a modified version of the engine from Metal Slug 3, which eliminated the slowdown problems of the original. In addition to increased difficulty, Metal Slug X introduced several changes to gameplay and presentation:
Much of the music has been remixed or altered.
A new announcer.
Many stages have the time of day changed. Instead of individual levels being set at either day or night, the stages can take place at dusk, twilight, or sunset.
It is now possible for the player character to become fat by collecting food items in every stage after Mission 2; in the original game, the obese transformation was only possible in Mission 4.
All levels have increased enemy count, new enemy placements, different enemy characters and new boss placements.
Vehicle types and locations are different.
There are more power-ups, POWs, and items (particularly food) in each mission. These items are often hidden.
Many environmental elements have different reactions when shot, such as exploding with unexpected results or randomly spewing out items or enemies.
Stronger versions of the heavy machine gun, flamethrower, shotgun, laser rifle, and rocket launcher are available. Each deals more damage than their normal counterpart, has a different appearance and a wider (or longer) area of impact. If the player happens to be fat while using these heavier weapons, they appear differently and can cause even more damage.
Several new weapons are included: Stones, Iron Lizard, Enemy Chaser, Super Grenade, Drop Shot, a new Golden Metal Slug, which is available in Mission 3, and the Armor piercer, available in Mission 4.
Added Fio's death sound (in Metal Slug 2, she uses Eri's).
New enemies are added or replaced, like the Mummy Dogs.
The original art for Metal Slug is shown at the end of the game while the credits are rolling, instead of the black screen used in Metal Slug 2.
《合金弹头 X》系《合金弹头 2》的升级版本，于 1999 年 3 月登陆 Neo Geo MVS 街机平台。该作采用了经过修改的《合金弹头 3》游戏引擎，彻底解决了原版存在的严重拖慢问题。除了整体难度提升之外，《合金弹头 X》还在游戏性与视听表现方面引入了多项改动：
大部分背景音乐都经过了重新混音或调整
更换了全新的游戏解说配音
多数关卡的时段设定被修改。关卡不再局限于白天或黑夜，还可在薄暮、暮光或日落时分进行
玩家角色在第 2 关之后的所有关卡中收集食物都能进入肥胖状态；而在原版游戏中，这种变身效果仅能在第 4 关实现
所有关卡的敌人数量均有所增加，调整了部分敌人的出现位置，新增了不同的敌人种类，同时改动了 Boss 的登场位置
载具的种类与出现位置均有变化
每一关都增加了更多强化道具、战俘与各类物品（尤其是食物），这些物品往往处于隐藏状态
许多场景元素被击中后会产生不同的互动反应，例如触发意想不到的爆炸效果，或是随机掉落物品、刷出敌人
新增了重机枪、喷火器、霰弹枪、激光步枪和火箭筒的强化版本。每种强化武器的伤害都高于普通版，外观不同，且攻击范围更广（或射程更远）。若玩家处于肥胖状态时使用这些重型武器，武器外观会进一步变化，造成的伤害也会更高
新增了多种武器：石块、铁蜥蜴、追踪弹、超级手榴弹、弹跳弹；第 3 关可获得全新的黄金坦克，第 4 关可获得穿甲弹
新增了菲奥的专属死亡音效（在《合金弹头 2》中，她使用的是英里的死亡音效）
新增或替换了部分敌人，例如木乃伊犬
游戏结尾播放制作人员名单时，会展示合金弹头的原始设定稿，而非《合金弹头 2》中使用的纯黑屏背景
