I'm looking to get the standard front sight/gas block removed from my AR and replaced with a Rock River Arms flip up front sight gas block.

Anyone know a gunsmith (preferably in Kendall COunty) that could do this work?

Gander Mountain has a gunsmith.  I don't know how good they are.  You can always send your upper to the factory.

In Oswego, try Tim @ Heartland Outfitting.  He works out of his home.  

You could just do the swap yourself.  If you don't have a set of punches, go to Harbor Freight in Aurora and pick up a cheap set.  Then all you need is a block of wood (1x2 works nicely) and a hammer.  If you have the M4 handguard you can remove it without any special tools.  Put the rifle butt stock on the floor and press down on the delta ring with your fingers and have a helper slide the handguard up and swing out (I'm done his without a helper too).  Installation is the reverse.

Swapping the front sight post is a 10-20 minute job.

If you feel like driving to Geneseo, I can help you do it some weekend.  I have a pretty tight schedule this weekend but next weekend would be great.

Let me know if you are interested.

Take care,
Bob