I'm in the process of getting ready to drill the take-down and pivot holes in an 80% receiver.  I have access to an OSI jig, but have already fitted the upper to the lower and the fit is very tight, very nice.  One of the castings I am working with appeared to have a shy E.D., it looked like if I drilled the holes, there wouldn't be much meat left around the hole.  This only was a concern on one of the ears for the pivot pin hole.  To compensate for that some, I removed quite a bit of material off the front face of the buffer tower to scoot the location of the holes back a bit, I wanted plenty of material around the pivot pin hole.

In the process of doing this, I got a really nice, tight fit between the upper and lower receiver, and think that's a good thing.  But now it's time to drill the pivot and take-down holes.  I don't have a lot of precision instruments capable of locating the position of the holes in the lower receiver.  I have considered this approach to drilling them without using the jig.  Let me know if it sounds plausible.  

Step 1:  Drill undersized hole through one side (side A) of the casting, increase the diameter, adjust location as much as I can while enlarging the hole to the point that it is well centered over the hole in the upper receiver.  Actually enlarge this hole (again, through just one wall of the casting) slightly larger than the size of the pin, actually large enough that a bushing would be needed to get a proper fit with the bushing in this wall.

Step 2:  Using the upper receiver to locate the hole in the opposite wall (side B), drill and ream the hole through the opposite wall (side B) of the casting.  This should assure proper location of this hole in relation to the upper receiver.

Step 3:  Plug oversized hole in the first wall (side A) with suitable material, probably bronze or steel.  Lok-tite in place.  The plug will be press fit into the oversized hole.

Step 4:  From the opposite side (side B) of the casting, where the properly located hole still exists, drill/ream through the plug in side A to properly locate the position of this hole.  The result would be, in effect, a bushing in one side (side A) of the casting and very accurately located holes.

This might work on both the pivot and take-down holes.  Does this approach sound reasonable?  Thanks!

It doesn't look like I've moved the upper so far back that I'll have those problems.  The lower is moved back about .020" now, that just gives me some extra meat around the one ear that looked a bit thin.  My big concern is if I use the jig, it will locate the pivot pin hole too far forward, giving me a shy E.D, and now, a large gap between the rear of the upper receiver and the forward face of the buffer tower.  Drilling the holes at the same time sounds like a great idea, I may try that on the other one I have.  I'd considered bonding a plug in the existing upper pivot and take-down holes, and using undersized pins.

that would probably work, but it seems like a lot of work. do you already own a 100% AR?  If you do you could very easily locate the holes by shimming the complete lower to the unfinished lower, then clamping them together with some c clamps.  then use locating punches lined up through your complete lower stamp the drill centers on your 80% piece. Then drill away.

Another alternative method (if you don't already own a lower) would be to make a template yourself, you can use either paper, cardboard, or metal. Lay a piece of flat material across the pivot and takedown holes on your upper then mark the holes on the material to be used as a template, then line up the template on your 80% lower and drill away. (this will only work on the pivot and takedown holes, to drill the other holes make your template big enough to put then on there too, and measure carefully mark, and place on your work piece.