Which should be installed first (I know the gas tube should be installed on the gas block and slipped over the barrel)?  the gas tube into the reciever and into the installed carrier or the gas tube then slide the carrier over the gas tube?  thanks in advance!

uuh the gas tube is "installed first" the carrier's key will be "installed" onto the gas tube each time you remove the bolt and carrieer for cleaning and each time the gun makes the bangy noise and cycles.

You should check the fit of the carrier key to the gas tube after installing the tube to make sure the tube isn't bent or canted, the key should slide over the end of the tube without binding or catching.

thanks!