While I agree with most of what was posted, my personal opinion is that the #38's are a dated concept for MOST rifle configurations. With products like the FF RAS, RAS II and SIR (and certainly with the upcoming URX), the long rail is obsolete for *most* configurations. Why? Because if you're going to add all that extra weight, you're better off with an integrated FF handguard setup (as listed above), which will give you a lot more performance per dollar spent, while also giving you everything you get from the Swan Sleeve.
For example: if I was making my own SPR, I would definitely use a FF RAS or a SIR instead of the PRI tube/#38EXT rail combo. While all three setups free-float the barrel and allow you to mount optics in front of the receiver, the FF RAS and SIR both provide significantly better barrel cooling than the PRI tube while weighing less. They also provide more versatility when mounting accessories.
-Troy