Disclaimer: Yes, I tried searching Google and Youtube. Making an account here for such a simple question was the next logical step.

With that said, I apologize if it's been covered. I tried searching here and couldn't find relevant information. (was only able to search last 30 days)

I've recently purchased an AR pistol with a nut at the end of the buffer tube instead of the castle nut that I thought all ARs came with. I even bought a wrench for the thing before it was shipped to me in anticipation of needing it to completely strip the lower.

Before I destroy this thing I wanted to make sure I was on the right track. The nut at the rear of the buffer tube--I'm assuming it's used to rotate the entire tube out of the lower. Is this correct?

NO! Turn the nut loose then Unscrew tube! Cover the buffer detent with your hand and catch it too

Pictures would help, but no, the nut doesn't rotate the buffer tube, it tightens it down.

Typically, you would loosen the castle nut all the way out so you can move the end plate out.  As it sits, the end plate is keeping the buffer tube from rotating, so it needs to be moved back.  

You may want to post pictures though, if it's not a castle nut, I'm curious what someone has done.

Remember, it doesn't have a castle nut. That would be easy to figure out since they're so common.

Unfortunately I don't have it with me but I found some pictures on the internet that may be able to illustrate what I'm dealing with

Usually pistol buffer tubes do not have the channel cut into the underside, to accommodate a notched end plate used on carbines.
Watch out for the spring and detent, once things get loose enough to allow the end plate to move.