It looks like I'm getting a 557 tonight, and I was wondering how much riser, if any, is needed to clear or co-witness a standard sight blade FSB?

I don't have the time or extra bucks right now to play ego games designing the perfect rifle right now, but I need to make sure that the sight is good to go (damned zombies!). Otherwise, I'd just swap out the unfashionable 14.5" HBAR upper on this A3 and get something requiring fore and aft BUIS. I do have a Pic rail adapter for the carry handle, which I might just use instead of grabbing a rear BUIS tonight.

Hope this goes through -- it's NIB for $350!

to cowitness an EOtech, it is good to go out of the box.

The fsb will be in the center of the EoTech field of view.  With a rear mounted BUIS, the sights will be "absolutely cowitnessed" if you zero them appropriately.  If you want the iron sights lower in the EoTech field of view, then you will need a short riser for the EoTech, for a "lower 1/3 cowitness". I believe the appropriate riser height would be about 0.300 in, or thereabouts.

I prefer the absolute cowitness, but its a matter of preference.  The line-of-sight for the iron sights is approximately 1.4" above the rail FYI.

As noted above, your GTG right out of the box, at least for a start.

Quoted: The fsb will be in the center of the EoTech field of view.  With a rear mounted BUIS, the sights will be "absolutely cowitnessed" if you zero them appropriately.  If you want the iron sights lower in the EoTech field of view, then you will need a short riser for the EoTech, for a "lower 1/3 cowitness". I believe the appropriate riser height would be about 0.300 in, or thereabouts.

In the case of the 557, it is already raised to allow a co-witness at the lower 1/3 of the optic window,
the newer designed sights have the new base with the 1/8 inch rise build in along with a improved thumb screw with build in recoil lug...






there is nothing required to mount the 557 to give the desired effect.....

On my DPMS with the 557 sitting on the flat top rail the holograph sits just above the FSB. I have an A.R.M.S L40 and it just clears the window so you can use the irons without taking off the 557. sits perfect for my needs. I needed the LaRue LT110 mount to do this with my old 512.