Just finished getting my rock river pistol together and went to mount a midwest industries single point sling mount to the buffere tube.  The mount is the type that you slide over the tube and tighten down with an allen key.  The problem is it wont fit.  I tried everything, opened it up as far as it will go and used lube.  Is the rock river pistol buffer tube a different diamater than a standard buffer tube?

If thats the case, what sling mounts will work with it?

Magpul

I use a single point attachment that fits on the butt end of the receiver.

IIRC the RR pistol tube is larger than the standard CAR civilian or milspec buffer tube to prevent one from being able to attach a stock.
Is there a weep hole in the rear end of your buffer tube?  
On mine I drilled and tapped the weep hole for one of those YHM swivel studs, the ones that rotate.  Works like a dream.  I cannot remember exactly the size and thread but YHM can tell you if you ask then it is a simple task to drill and tap then blue lock-tite the stud in the hole.  Use only a little lock-tite or it will draw into your stud and it will no longer rotate.

The RRA is a larger diameter.
So I sold it and purchased an Ace pistol buffer tube.