M&P gas piston shoots low

[nbfr715]

Recently, I purchased the M&P 15 gas piston with quad rails and bought a pair of yankee mills iron sights. I tried to sight the gun in and have dropped the front iron sight as low as possible but keep hitting the target 1.5-2 foot low at 60 yards. (and no, I'm not just a bad shot)
Does anyone have any suggestions about what to try next?

 

[Super Trucker]

My guess would be that you need a shorter front sight, if you are sure it is not operator error.

 

[nbfr715]

bought a gas block mount front site instead of a rail mount.

 

[Dragon88]

Shorter front sight or a little riser for the rear sight.