I have a Stag Model 7 (Hunter 6.8 SPC). I know very little about AR platforms. My understanding is that you can buy "uppers" and convert guns to a different caliber without much smithing knowhow.
Can I convert this to a .308 or even a .223? If so, exactly what would I need to get to complete this mod?
I have some skills working on guns. I also have a good friend who is VERY knowledgeable in smithing.

The 6.8 and .223 calibers use the same lower, and you just change out the upper by pulling 2 pins. Very easy to do.
The .308 requires a different lower made to accept the longer cartridges.

Oh sure . . . that makes sense. So .223 would be more practical/affordable.

To convert a 6.8SPC to a 223rem/5.56, you can either swap uppers and magazines and just do an "upper swap" that is temporary, or you can swap the bolt (not bolt carrier group), barrel, and magazine to do a more permanent swap.

Depending on the deals you wait for and what you want in your upper, sometimes it's cheaper to find a cheap complete upper than it is to buy a barrel, but more often than not, if you're comfortable swapping barrel (honestly not difficult if you have a torque wrench, armorers wrench, and vice), then you usually buy a barrel cheaper than a complete upper.

if you want 308 capability in the AR15 (the 308 is a longer action AR-10) you could get a 300 Olympic Magnum, but you need a complete upper and must reload, also you could get an upper in one of the wssm's, I have a 25 wssm and its one helluva deer rifle
RR