How do you use the P4 Sniper Reticle?

You point it at some one and pull the trigger then the gun goes BOOM does that answer you question?

Stay out of the way. Shoot people from a place where they can't shoot you back. Shoot a lot, and do a lot of damage. When they attempt to run away, assassinate them with your ultimate. Desolator is a...

there isn't one answer here, it depends on the weapon and what the shooter is comfortable with, another thing to take into account is cost

Well, many. But an example is the m14.

They're drawn from the ranks of the infantry and cavalry. There are some in EOD, as well, although their purpose is to detonate ordnance, rather than to engage the enemy.