I already set SpreadRange = 0, Separation = 0 and almost anything that would make the bullet spread.
Sometimes when I shoot with this sniper rifle, a bullet goes as spread as a shotgun pellet would.
Help?

ShakeRange controls the accuracy when not aiming, the less the better the accuracy
SharpShakeRange controls the accuracy when aiming, the less the better the accuracy
ParticleSpreadRange controls how much the different particles shot at the same time will spread, a high value will make a shotgun, 0 will make a powerful bullet(provided your gun shoots more than 1 particle per shot)

also here is a complete list of variables used to make a gun with their description:
download/file.php?id=35285

You want ShakeRange and SharpShakeRange, under the HDFirearm entry. Smaller numbers are more accurate. The first one is for hip fire and the second one is for sharp aim fire. NoSupportFactor is what to multiply the numbers by when the gun is held with one hand due to being wielded with an off-hand item or when the support offset is too far for the actor to reach. Separation is something else.
EDIT: Ninja'd

Ok, thanks for the info. Now I'll make a pseudo-overpowered sniper rifle. Notice the pseudo.