I've got a PTR 91 A3S that I bought from someone here on the EE. It was in unfired condition and had been upgraded with a steel lower and a wide hand guard. I finally got a chance to take it to the range today. The rear sight drum was very difficult to turn. The rifle was shooting low so I went to adjust the rear sight. I was using a pair of snap ring pliers to depress the detents. I could only turn the drum about 1/2 a turn before it basically froze. It feels like it might be cross threaded?! I can't figure out how to completely disassemble the rear sight? Any ideas what could be binding or any ideas how to completely disassemble it?

It is riveted together and therefore not easy to disassemble (or re-assemble afterwards). I'd soak it in Kroil and see what that does for you. RTGparts sells a dedicated sight tool.

2nd on the Kroil - very good stuff.

I had one that was very tight as well. I just ordered a new sight from RTG. Easier than trying to loosen the one I had.

http://www.robertrtg.com/store/pc/HK91-G3-REAR-SIGHT-SURPLUS-27p2102.htm

Also make sure you are rotating the right way. They are standard threads, not reverse threads. So if the outer sight drum is stationary you should be rotating the innner clockwise to raise the rear sight - raise point of impact. If you are shooting low now maybe you were rotating the wrong way and bottoming on the base? Just a thought.

I went ahead and ordered the rear sight from RTG last night. I had a PTR 91 several years ago, it was a JLD, so I'm fairly familiar with adjusting the sights. When I take the rear sight off, should the detent ball under the base come out? I could not get it out if that's indicative of a problem.

Sometimes the ball will fall out; sometimes it can't.

You did the right thing ordering a new sight. With the one I had after I put the replacement one on that I received from RTG I soaked the original one in Kroil for about a week. After the week it still would not budge.

The ball should come out - one way or another.