The Army tests with the M68 CCO sight (Aimpoint CompM) demonstrated the farther from the eye the easier it is to pickup the dot and transitions between multiple targets is faster. Probably because shooters are already accustomed to focusing on the front sight. If you shoot the accepted best "stock weld" of nose to charging handle the sight acquisition and consequent dot acquisition is automatic.

Individuals may find different locations work better for them, but far forward sight and nose to charging handle is the place to start for the vast majority of us.



On the M&P15-22 with a full length rail you can mount it right behind the rear sight.

Here's my pair (my old primary Bushmaster 5.56mm M4 Carbine and 4¢/rount M&P15-22 trainer):



On rifles with rails only on the receiver push the sight out as far as you can.

Once you find the best configuration set up your duty ant training rifles the same way.

-- Chuck