I removed the barrel nut from my upper and slid it down the barrel to keep it out of the way.  When I tried to slide the barrel out of the upper it wouldn't budge.  I tried a few smacks with a rubber mallet on the upper just above the gas tube whole, but it still wouldn't come loose.  The barrel extension just slips into the upper with the index pin in the slot, right?  Since the index pin prevents "twisting" the barrel to loosen it, how can I work it loose without damaging the upper?  I thought about sliding a dowel rod through the upper until it touches the barrel extension, with the upper in a vise block, and then trying to drive the barrel out by hammering on the dowel rod.  Is this a good idea?

You are correct in that the barrel does not thread onto the upper receiver. The extension just slides into the receiver and lines up with the slot.

Sorry, but I do not know the best way to remove it, but I am sure someone here does. Mine always just pulled out by hand.

Try this, with upper positioned in Action block and in vise (tight), use a small wooden hammer and slide the end of the handle into the inside of the front sight tower. Whack the head of the wooden handled hammer toward the FS or end of the barrel, in as straight a direction as possible, away from upper.

Mike

Try the dowel rod thru the upper first.  If that don't work, try a heat gun/hair dryer/propane torch aimed at the joint; heat it evenly all around, the aluminum upper will absorb heat faster than the steel extension and expand, then a good wack with the dowel should do it.

If you want to get the most out of the heating technique, I would first put the parts in the freezer over night. Then apply heat to the upper. The aluminum upper will expand much faster than the cold steel barrel. Dave C.

Before using any force or heat on it, I would try Liquid Wrench or, better yet, Kroil, and give it an hour or so to work. I've never had to use either a hammer or heat to separate a barrel and an upper yet, after using Kroil.

Problem with using a solvent first and then heat later is potential of flashing and the stuff turning into goo with the heat and then solidifying.  Note that some competition guys use Loctite on the barrel joint during assembly with a laser bore-sighter; the barrel nut is put on after the Loctite has cured.  I haven't had an AR barrel yet that couldn't be removed with a bit of heat and hammering.