I can't pull it off by hand and I don't see how I could tap it off with a rubber mallet or anything.

This is after you've removed the barrel nut right? (Just making sure)

If the barrel won't move, take a wooden dowel and put it in the upper receiver. Make sure the dowel covers the majority of the barrel extension. Tap the dowel with a hammer and the barrel should come out.

Stroke it and call it baby!  LMAO

A rubber or rawhide malet on the rear of the FSB if the upper is in its fixture and nut unscrewed.Or a piece of wood behind the bayonet lug and a hammer or malet after the nuts been unscrewed

Put the upper back on the lower,insert 1 round, pull trigger...voila!
Barrel off.....

Seriously, the tap with the wooden dowel works great.

Some penetrating oil sprayed around the edges and let soak overnight can help.

Heat, if you are very careful, may help;  Stick the muzzle end of the barrel in ice water, take a plumbers propane torch to the front of the upper receiver, just a few seconds at a time, you only want to heat the receiver up some I would think you would NOT want to go past 200°F, you don't know if that will damage it.  As you are probably guessing, this is big time risky, it wouldn't take much misunderstanding or inexperience to destroy your upper receiver with a torch in your hand.  I'd only do it as a last resort, and then only with someone with some experience using a plumbers torch (a clean burning torch used to heat pipes for solder without damaging the pipes).

Use a little anti-seize rubbed into the contact surfaces of the barrel extension and upper receiver when you put them back together, so they seize together the next time.