If you are looking for the weapon, see Future Soldier.
Future Soldier is hidden character in SFH 3 who is reference to Raze series.
He is dead soldier whose pieces can be found in several maps. By interacting with those pieces you can earn secret medals.
On the Gorge map, there is his helmet on the highest rock. You can't get up there because it's too high but if you blow yourself with an explosive weapon you can fly to the sky and reach the helmet and you get the achievement and a new game mod.
On the Ice Caverns map, there is his frozen foot sticking out from the ice rock on the right side of map. You will need a Flamethrower to burn it. After few seconds it will be melted and it will disappear. You will unlock an achievement and new game mod.
On the Volcano map there is his hand near the lava river in the right bottom. You need to jump to it and hit it with melee weapon. This also means you die in lava, but you will unlock new achievement and game mod.