I picked up a set of the DPMS magonel flip up front and rear sights at Gunstock and they are of very good quality. They lock in place very solidly.
I shaved the front gas block and installed a quad rail free float system I also got from DPMS. The flip up front sight sits on the front of the rail. I used a 0.040" taller front sight post because the quad rail sits marginally lower than the flat top upper. There is another setting for the front sight if it is used on a picantinny gas block.
I sighted it in and managed to ring the 12" diamond steel plates out to 200 yards with the Mangonel BUIS.
Rifle with my Elcan attached. The Magonel sights fit nicely underneath it. I think they would work with an ACOG as well, but I don't want to take mine off my 20" rifle to try it out.
The Magonels are a nice system especially when used together, and offer a good alternative to other BUIS on the market.