I just received my dissipator barrel from Bushmaster today and noticed that the handguard cap is very loose.  It has a roll pin attaching it to the FSB (the roll pin is sticking out over 1/4"), but it still wiggles so much that I can almost pull it off the roll pin and slide it down to the gas block.  I called BM to see what they suggested and the rep said that once I put the handguards on it will be good and tight.  I have never seen a barrel with a loose handguard cap and do not fancy the idea of everytime I replace the handguards, having to chase the cap around.  Should I hammer the roll pin in more to try to tighten it up?  Any ideas would be greatly appreciated.

Normal but a hassle, if you don't like the cap wandering try some red loc-tite or super glue. If the roll pin in gas tube hole is bottomed out, try flaring the end out with a center punch to retain the end cap.

You've never seen a loose handguard cap before because most barrels have a a raised section behind the FSB that holds the barrel cap against the FSB.  You can't do that on a Dissy because a raised section would make it impossible to slide the shaved gas block into place.

I believe the roll pin is simply inserted into the useless gas tube hole in the FSB.  You could use vise grips to pull it out then tap the hole and use a screw to hold the cap tightly to the FSB.