I have a 16inch bull barrel carbine with a picatinny gas block.  I put an ARMS 40L and what I believe to be a YHM fixed front on the gas block. My problem is that the front sight is too high as I can't screw it down far enough to get a 50 yard zero.  I am still high with the post bottomed out.  

Is the YHM not compatible with an F height front sight?  Is the Bull Barrel configuration of my gas block higher than a standard railed block?  Do they make or do I need a different gas block?   Does anyone have a suggestion?

One more thing to add to the mix, since I plan a railed FF forearm, should I just order a front sight for a railed forearm and be done with it?

OK. to answer my own question, I just took apart my front sight and found why I could not turn it down far enough.  There was a burr keeping the post from lowering.  I now can lower it quite a bit further so I will try to zero it again with these sights.