I bought a carbine FF tube off EE and the existing stud indexes at about the 2 o'clock position (when viewed from the rear) when tightened up on my upper.  

Question:  Are there shims or washers specifically made to index screw-on FF tubes correctly so that the stud properly locates at the 6 o' clock position?

I suppose I could just drill/tap another one and fill the existing hole with a set screw.

Quoted: Question:  Are there shims or washers specifically made to index screw-on FF tubes correctly so that the stud properly locates at the 6 o' clock position?

None that I've ever seen.  Closest thing would be shims to reindex the barrel nut itself.

What type of tube is it?  Did it come with a locking ring for the tube to fit over the barrel nut?  Or is this a one piece type?

Some pictures amy help if you have the capability.

Just the tube and barrel nut.  It's about as basic a FF tube as you can get.  I'll see if I can get a picture to help show the problem.