I'm looking to extend the bottom rail on an M4 Commando with an older Midwest Industries rail system.  Non-free float barrel with a standard front site aperture.  Viking Tactics Ultralight Vertical Grip mounted on the bottom rail.  I just want to extend my support hand a bit more since I'm a taller guy and the shorter barrel doesn't allow for much of a rail length and I don't always have a chance to wear some gloves to protect my support hand.  I haven't been able to find a product that "shrouds" the underside of the barrel.  Any and all suggestions welcomed.  Thanks in advance.

There are a few companies that make an extended quad rail with a cutout for the fsb, maybe that would work for you

It's a company gun, so I can't do certain things.  Do you know whether the rail would leave the handguard cap intact?  I'm trying to picture how that would work without making it a free float.  Also, any specific manufacturers you have in mind?  I have been unable to locate any on my own.  Thanks.

Midwest Industries makes a drop in one, still uses cap at fsb.

http://www.midwestindustriesinc.com/index.cfm?fuseaction=category.display&category_ID=422

Here is the Midwest one.

How long of a barrel, and how much modification can you do?  

Something like the Diemaco, YHM, or Ergo barrel mount add on rails with some rail covers might be the ticket for you if you have enough barrel and can't do much modification.  

~Augee

11.5" barrel.  However, the good news is that the boss just gave me permission to remove the FSP.    So my original problem is no longer an issue.  I'm going with a Midwest Industries free float tube and gas block.  Thanks to everyone for responding.