How do I know if I need a riser rail?

[dashadow]

I think I posted this in the wrong section, so I will try here.

Just bought a Sight Mark Laser Dual Reflex sight for my AR, and was wondering if I should mount it right to the flattop, or should i use a riser rail? How would i know if I need one? How high, etc?

Thanks in advance.

 

[happygeek]

Umm, you really just need to be able to see over the front sight post, that and have it just high enough to allow you to get your eye behind it, the AR stock is higher than most rifles due to the buffer spring being in it. If I'm not badly mistaken, you're best off having the sights as close to the barrel as you can possibly get them.