A guy has a nice Mossberg barrel for sale cheap, but it's real tree camo and my gun isn't. I assume it's hydro dipped? How would I strip it and what's under it? Did they blue them before dipping?

I just used grill high temp flat black spray paint from walmart. Stuff worked great and looks parkerized.

I suppose you could try acetone, but be aware that it may only take part of the dip off, and that the barrel could potentially look worse. Maybe try a little on an inconspicuous spot before going all in.

A little late to the party but figured it's worth a shot. I've removed hydrodip on many dozens of guns and each time it's as much of a pain in the ass as the last. Acetone is a must. Submerge the metal parts in acetone for 10-15 minutes (plastic parts too if you're brave or just scrub them with an acetone soaked steel wool pad) then either take steel wool (any grade) soaked in acetone or sandblast (works best if you have access to one) to the parts. You will likely have to soak, scrub/blast, soak, scrub/blast until it's all done.
As someone stated earlier, the underlying finish will probably suck so be prepared to refinish with anything from BBQ grill paint for that classy look or Cerakote.

Depending on how the hydrodip was applied at first, how it's been treated since, and the age all affect the ease of removal. I've had a few that required very little scrubbing, others took hours. You will likely have a good bit of hydrodip flake off after a couple minutes in acetone, but what doesnt flake off will probably take some elbow grease.