I am building a Flat top and have an F Marked Front sight base. I am looking for some Ideas on rear BUIS for it. I am not sure at this point if I want to go the flip up route or not yet so give me all the ideas you have. Most of the product descriptions I have seen don't say if they are for F height bases or not. Any info would be great.

Troy Industries folding rear sight will do the trick.  It locks solidly and when folded they fit nicely under most optics.

They won't be marked F or not because it doesn't matter. Most should be set to a pretty universal height. The elevation is adjusted by rotating the front sight post left or right, raising or lowering it.

I know how to adjust but the F marked base is (I think) .04" taller than the standard base at the flat portion. I have heard of several people having problems of not being able to bring the front down enough for some rear sights. I have seen a few that are listed as being for standard front sight bases but have not seen any that state that they are for F marked bases. I was thinking that if the rear is adgustabl for elevation and windage then I may be able to compensate for the difference.

You'll never really know if the front sight post will have enough adjustment room or not until you try. Not all AR's are created equal. I haven't seen that problem around here very often, and never experienced it my self.

Usually people end up having to raise the post too high because the FSB isn't F marked(they make a higher front sight post to compensate for this).

I've looked around a lot of online shops since joining here a looked at just about every FSB and never seen one listed as being specifically for an F marked sight base.

I'd get one you like, can afford, or fits your needs. Though I'd stay away from anything that doesn't include a brand name in the description.

If they did, it had nothing to do with the "F" front sight base or any of the quality rear back-up iron sights (LaRue, Troy, KAC, ARMS, GG&G.) The front sight post on a Colt front sight will bottom-out in the sight well at approximately 24 clicks below flush. On an “F” marked front sight base the sight “shelf” is a whopping 0.040” higher than on a standard front sight base. That means you might have to turn your front sight post down approximately 6 clicks further with an “F” marked front sight base than on a standard front sight base. (6 whole clicks difference. Oh, the inhumanity!)

I have two Troy rear flip up BUIS for my carbines with F type FSB (one was marked the other not marked but both are 0.040" taller) because sometimes I use a magnifier and higher power scopes.

The Troy are better in my opinion because of its solid feeling either raised or folded, no movement at all when being used, the windage adjustment are set in to prevent accidental mis-adjustment, and a low 0.460" folded height that would fit most scopes.

I would recommend the folding BUIS type so if you get a magnified optics later on you don't have to sell your fixed BUIS for a folding one.

If you want a military style BUIS take a look at the Matech, KAC, and ARMS, etc., the ones with the lower folded heights are the KAC and the ARMS model # 40L.


The Big Bad BUIS FAQ - tacked thread link

Great... Thanks for the info. I guess I may have not understood the info that I had heard about the problems others have had. I will look at the troy and a few others but I will lean towards the troy.

Thanks

As stated, you won't know if your BUIS will regulate with your front sight until you try it out.
If you can't gain poa/poi with the sight combination, there are taller front sight post available from Bushmaster, DPMS and I am guessing about everybody else who
offers stuff for AR15/M16 rifles.