You could put a aluminum heat sink on the barrel:
http://jprifles.com/photos_new/heatsink.jpg
Then leave it open on the back end and then extend the barrel shroud beyond the muzzle so the muzzle blast causes air to flow through the fins and cool the barrel... Wait, that's been done. Worked pretty good at allowing for a higher sustained rate of fire, but the shroud got pretty hot as well....
http://photos.imageevent.com/badgerd...ld%20Mount.jpg

