I've got a Troy BUIS on my carbine, and I have my front sight post set for the lowest elevation possible.  Still, my impact is about 4 inches low at 50 yards.  

Anyone else have this problem?  I really don't want to put a riser on the BUIS, but I can't think of any other way to get the rounds up.

The other option, of course, is to try a different BUIS and see if the problem is related just to this particular one that I have.

Anyone have any thoughts?

Thanks, and Semper Fi

ETA: I'm talking about the rear folding Troy BUIS.  Not the front sight.

you could buy another front sight post for 5 or ten bucks and file off about .030

I had this problem with a Bushmaster superlight barrel.

Wasn't using an upper that required BUIS so I ended up switching upper receivers. Problem was with the barrel. Didn't bother measuring to find out where it was off - just filed the front post down a bit and it was GTG.

Thanks Guys.  
I was afraid that might be my only option (besides a riser).  Guess I'll have to get out the ol' file!