The two most reliable setups are the drill and tap mount Choate and some of the various Russian setups or the scout mount as mentioned above that replace the rear sight leaf. Any mount that is attached to a moving part,gas tube or rear cover will generally not hold accuracy of time as the part moves or wears.
Being your reloading as I do for the X39 you want to use a mount that is rock solid to take advantage of the increased accuracy of you handloaded ammo. Any one of the above mentioned mounts will provide that all the rest is just the luck of the draw and anything that you have to remove and replace each time you clean your rifle will require you to rezero the scope or red dot. For the drill and tap mounts a scope works well be aware though that most of the rear mounted scopes of any quality are long and want allow you to use stripper clips to load the rifle. I find a good red dot such as the Bushnell or Tasco pro pointer design that uses the rings to mount to the rail on the Choate or scout mount to be the best also pay attention to the dot size the smaller the better 3 MOA is good up to 100 yds. Anything larger just covers more area on the target.
Also on the Choate mount the base mount red dots like on the 40+mm sights want fit the rail because of it's delight that why you need to use the one's with the ring mounts. On the scout mount you can use either.
I use a Choate on my Yugo