Sniper: There are 3 methods of using the sniper: first being the 'snap' reflex, which is a form of getting headshots (snipes); second is to aim for the torso, which is reliable but less effective, especially when you might as well develop the muscle memory/ coordination for the snap reflex; and the absolute aim, where you learn to pull the reticle directly toward the head (someone familiar with quick-scoping from CoD could try this).
-The snap reflex, or snapping, means to flick or slash the reticle across the enemies head. The aim magnetism pulls the bullet toward the head and delivers a fatal blow. You may do this by accident and brush the occurrence off as 'luck' or fail to recognize it over the 'cursory' movement, but try practicing this from time to time.
-You should only attempt to shoot for the torso when the enemy is running at you, which is also when headshots are easiest. When they are faced sideways or at a considerable distance, shoot for their head. This will lead to more kills, holes in heads, and satisfaction.
Rocket launcher: Watch out for the blast radius. You'll either kill, be killed, or both; within an uncertain ratio. #careful
Railgun: Zoom in when nearly fully charged to snipe. It takes minimal practice and is essentially no different than zooming in with the DMR for an instantaneous headshot. The guns works quite nicely with dexterity and thruster pack . This can be acknowledged as Halo 4's second most competitive power weapon, regardless of what other nay sayers may dish-out. Reason being is that there are effective ways of combating this gun's onslaught: breakneck strafing and cover. Few other power weapons are given this 'competitive' edge, besides of course the sniper rifle. The beam rifle is OP.