Hey Guys:
I was looking at the BUIS FAQ threads and wanted to find out if anyone ever did a compatability study on using fixed FSB bbls with different vendor flatops.  I have both Bushy and Wilson barrels, neither of which will zero correctly with my flatop uppers from Delton (they both shoot approx. 2" high at 25m, with the FS flush with the well).  I can go with an extended front sight to compensate, but would prefer to keep my Trijicon FS.  So I am looking for a BUIS that will work with my rifles.  The GG&G A2 BUIS doesn't work.  It appears to be too tall.

The usual cause of the problem are using A2 FSB (which are low) with flat top upper receivers.

Most of the rear BUIS and removable carry handle for the flat tops are the same.

Some manufacturers are doing this and the customers get surprised when they zero and usually just buys a taller FS to correct the problem.

Other mfg that are doing it the right way are using the correct FSB with taller shelf about .032" - .040" taller with standard size FS.

Colt marks their FSB with the letter F to distinguish it from the A2 FSB but others like LMT don't mark theirs but they are the taller type.

If you want to use standard size FS you could get a two piece flip up FSB like the YHM or a picatinny gas block with fixed removable or flip up BUIS that uses standard size FS.

Another way is to get a tall type FSB that's blank - no holes for the taper pins and have a shop install it, it's a complicated and expensive work because the original taper pins holes under the barrel have to perfectly match the new holes of the FSB while making sure the sight is at top dead center.

Hey thanks for the reply.  Looks like I'm just gonna have to go with the extended FS to make my rifles work, until I shoot out these barrels!  Dirty work to be sure, but someone has to do it.