I used a 1/2" OD round flexible foam insulation from Home Depot (used for weatherproofing doors) and wrapped it around 14 times (tapering the start and end points). This made the whole thing 7" long. From the screen-caps I had, this appeared to be correct (to me, at least). I just used hot glue to attach it. It really holds well! But be careful to not get the gun too close to the foam or else it'll melt it.
Then I painted several layers of black "Plasti-Dip" over that. This stuff is GREAT! It holds well and is really flexible... so I don't have to worry at all about it chipping off.
i still need to weather it a bit... I'm new to guns n' stuff (I'm more experienced with costume making), so I dunno if it looks ok and is up to MEPD standards.
Edit: Here's the foam I used: http://www.homedepot.com/p/Frost-King-E-O-1-2-in-x-20-ft-Poly-Foam-Caulk-Saver-C22H/100159362#.Uqfp9BkpDjM