[Vertiviper]

Do most rear flip up iron sights work with a standard A2 front sight? Or, will the front sight be slightly higher than the rear?

[hillbilly68]

The BUIS concept was designed for just that purpose, to provide a backup to use with the standard M4 front sight assembly when an optic is used as the primary.
Im unaware of any rear that doesn't align, the ones I have experience with change the front plane (eg a gas block installation), but they still align correctly.
But that is just my limited experience with them (MI, Troy, YHM). Some of our more well rounded guys may have better data.

[SGT-MILLER]

The standard BUIS sight will work with the A2 "A-Frame" front sight post.

If there is a certain maker out there that makes the BUIS by different specs, and will not allow it to align with the A-Frame, I don't know about it.

Dillinger, where you at????????????