Let's say I found a "real deal" on a 16" upper 6.8 SPC II chamber, all cool, 1:11 twist with 4 grove rifling, SPC II chamber and it's in a carbine length gas system.  Can one just plug the existing hole on the gas port and drill a new  port up the barrel a little further to a mid-length position?  Of course you woud have to change the gas tube, or in the case of a piston build, use the appropriate length piston system.

Sounds like a plan to me.  Just make sure the old plug is secure and won't blast loose under 10,000 psi or so of pressure - tapped and plugged or plugged and welded over.

What is the barrel diameter where the midlength gas system would go?  Is it a common diameter, or would you have to make or modify the barrel or gas block to make it fit?

Hypothetically speaking, where might one find a real deal like this if one was so inclined?

The area in front of the current gas block is likely smaller in diameter.  You'd have to have the barrel turned down to .625 to fit an A1 gas block or have a custom gas block made.

Good points.  The area in front of the existing gas port would be desirable to be the same dia.  More than likely, as I don't have barrels at hand to look at, but I would guess the best candidates to move the gas port location would be makers that also make a carbine length and mid length system in their same respective models.  CMMG comes to mind as one that has either or.   Otherwise, yes, if you have to have the barrel turned down a bit along with the respective hole drilled for the new gas port on top of plugging the old port, the extra expense may not be worth it.  In other words, you may "find" a new 6.8 upper and get a very good deal on it an then be able to trade the upper or the barrel to a dealer, and even kick in a bit for a different mid length gas ported upper/barrel, and come out better financially.  This is assuming new unfired condition.  If there was no other taper in the barrel, or at least there would be no additional taper where you would move the gas block forward, then it might be a workable and not too expensive option, yes?

i'm pretty sure ADCO could do it but I have to ask why?  Sounds like it would be easier and cheaper to just build it with carbine length gas system.  I realize the midlength offers a longer sight radius and decreased chamber pressure, but by the time you spend the money on it to have it remade into what you want, is it still going to be such a steal on the barrel?  I'd just build it as is.  just my 0.02

^^^I would go with this idea too if I could find a gas block that fits nicely inside of free float rail or hand guard.

But I can imagine the reasons he might not want to go that route because of an A2 or flip sight/gas block or slower round cycling?