Neutral Special: Big Boss pulls out The Patriot, the gun given to him by his mentor, The Boss. It’s an assault rifle so it shoots at a quick pace. You can move while shooting it, but you cannot jump. It has good range but doesn’t make the opponent flinch, so it’s good for racking up damage if you manage to trap an opponent against a wall or such. You need to reload after shooting it for a while, but you never run out of ammo or have to pick ammo up, because it canonically has “infinite” ammo!
Side Special: Big Boss yells "ROCKET PUNCH" (or rarely, "ROCKET PUNCH" in Japanese) and taps his bionic hand, which launches forward. It goes at a steady pace but you can make it go faster by holding the special button. The hand is very quick in how fast it goes when you input something so it's hard to reach the hand behind an opponent, so it's very good to keep an enemy away if they're right infront of you.
Up Special: THE FULTON. This is one of Big Boss’s wackiest and coolest moves. It’s also very unique.
If you’re standing next to an opponent, the fulton is attached to them. They’re suspended in the air, and can wiggle out of the fulton, but if they don’t in time, they blast off into space, most likely netting a Star KO.
If you’re next to no one, then Big Boss ties the fulton around himself and is suspended in the air. He shoots upward, but not too fast; fast enough that if you have decent reaction timing you could easily recover from off the stage. Also, if Big Boss does this in the air, the fulton is still suspended, so you can meteor smash him.
Down Special: Big Boss throws down a Decoy. It stays on stage for 5 seconds, and if attacked, explodes. It takes a few seconds to set up the decoy, but if you can set it up well, then you have an easy KO on your hands.