Hello Fellas,
I have just aquired a Unertl Tube Sight and I don,t know anything about them . How do you use one ? Do you use the front aperature sight on the barrel as the front sight ? I am not sure if the one I have is complete or not. It is about 18"long and has very well made Unertl mounts on it .
 Thanks in advance , Bison-j

Your Unertl should have the baffled tube (18" as you noted), the same front and rear micrometer mounts as their scopes, and a threaded eyecup at the rear.  Some people substitute adjustable apertures for the eyecup.  

The advantage to this type of sight is having the same sight line (cheek position etc...) for both scope and irons.  Some people also like the sight picture, although the restricted field of view can lead to cross fires.  Another disadvantage is that the sight line is fairly high.  It is really just a personal preference thing.

If you have external adjustment scope blocks on the rifle already, the only fitting issue is likely to be with the front sight height.  You will probably have to adjust that to center up the sight at the distances we use.  If the front sight is mounted on a scope block, that shouldn't be much of a problem.  If it isn't, you might have to buy or fabricate some sort of riser.