You say you can be off by 0.005" or even 0.010" every 4". And people here are talking about using jigs with that 4" hole spacing to successively locate each hole. Have you given any thought what that could add up to in 14 feet.
14 feet = 168"
168" / 4" = 42
42 - 1 = 41
And 41 x 0.005" = 0.205"
And that is with your LOWER figure. It could be twice that at 0.010" per each 4".
I don't know what size screws you are using, but 0.2" is greater than half the diameter of a 3/8" bolt. Your hole would need to be made as a slot or with a diameter of 3/8" PLUS 0.205" or 0.580" to guarantee that the screws/bolts could be inserted over the entire 14 feet.
If you are using a #10 screw, the hole would be close to the size of the head or even larger, depending on the type of head used.
I am sorry, but you probably need that +/- 0.005" tolerance over the full 14 feet. Or at least +/- 0.010" over that distance. With that in mind, I do not like the idea of using a 4" jig to locate each hole in succession. Or, if you do, then it needs to be 4" +/- 0.0002" and that includes the slop when it is used. Even if you ream each hole, that is a tough nut to crack.
I would:
1. Try to make my first hole in the center. That way you can work both ways outward from it. Depending on your machine's requirements, this starter hole may possibly only need to be roughly located. Perhaps as much error as a quarter inch would be tolerable. But you must answer that question.
2. Use several jigs. The first one would locate on that center hole and have guides for two or three more on each side of it. This jig would be used for all of your rails and would be made with as much precision as needed, perhaps +/- 0.001" over it's whole length.
3. Then a second jig would be used for the next nine or ten holes on each side. It would have at least four pins in it so it will locate itself using the original center hole as well as three more on the one side and zero to three on the other side. This jig would be used on both sides of the center to bring the hole total to 5 + 9 + 9 or 23 total holes. This second jig would also be made to a tight tolerance, perhaps +/- 0.002" over it's full length.That would bring our total error to a maximum of +/- 0.005" over the center 23 holes.
4. Finally the second jig would be moved to locate itself on the outer 5 (or 7) holes made in step 3 and the remaining holes would be drilled. At most this would add an additional +/- 0.004" making our error total +/- 0.009" which is just under your original high figure for every four inches.
If the tolerances still add up to too much, you can tighten them up for the second jig and even for the first. If the same jig can be used for both the rails and the supporting frame under them, then you can probably loosen up the tolerances on both jigs a bit.
Part of the idea here is to use more than one hole to locate the second jig. That way, any errors in those holes used to locate it would be averaged out to a better figure than could be reached when using only one hole.
Of course, a nice CNC drilling machine with a 15 foot bed would be absolutely ideal.
.005-.010 every 4".
That's kind of what I was figuring, but just making sure someone didn't have a better idea.
The 1/8" wall thickness works fine, but thanks for checking.