Will a troy 9" hand guard cover a cut down front sight gas block.

Carbine length I presume? It will definitely fit over the gas block. Measure 9" from the upper receiver and see if it goes over the gas block or not.

I think he means will it Lear the gas block. It should.

 

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It should as long as you trim it properly. I have a alpha and bravo rail both on cut down/trimmed FSB's.

Yes. I have used Troy Alpha rails on 3 rifles where I shaved the FSB down.

The 9" ends right at the muzzle end of the gas block,  Most likely will go with a 11". 

If you buying an Alpha tube to put on a 14.5" barrel pinned to 16" go with the 13" tube. Make the long barrel work for you by giving you more room to grab the gun further out. The closer to the muzzle you can get your hand, the better you can drive the gun.