Wow, I really appreciate all of the input that I've gotten - lots of great advice!
I've been thinking about it and considering all of the information on here and I have narrowed it down to 2 options:
1. Get a SCAR 17S.
2. Build my own .308 piston operated AR-10.
Both have different appeal to me:
The SCAR may not have my 1 MOA requirement once I get it out to 300 yards, but from the research I've been doing, as long as I do my part, I can keep it at 1.5 MOA and better. And I can deal with that. As far as weight and reliability go, the SCAR fits my requirements perfectly.
Building my own AR-10 I'm pretty positive I could meet all of my original requirements, including budget, except 1 (now 1.5) MOA. I could do this... but there's really no way for me to know what kind of groups I'm getting until I get the thing put together. If something is off with my build I might be at 3 MOA and have no idea if I did something wrong in my build or if I did everything perfectly and my gun just isn't capable of the kind of groups I am happy with.
So I guess at this points I'm leaning more towards the SCAR. But in order to do this I would have to live with just the iron sights for probably 6 months or better (until my budget allows me to get some quality glass)
If you build an AR-10 with good components, especially a good barrel, then there's really not much you can do wrong. AR builds really are that foolproof these days. The only way you could really go wrong is if you use an el cheapo upper receiver that has a lot of slop in the flange, and even then, while not ideal, you can still bed it and make it work. Long story short, go with the BHW barrel in an Aero upper receiver and you're almost guaranteed truly exceptional accuracy at a very reasonable price.
I think you're going to encounter a compromise decision with the piston kits, though. If you use a piston on a free floated barrel, then the movement of the piston is going to interfere with barrel harmonics, so you either have to get a heavier barrel or just live with the reduction in accuracy. There's really no predicting the outcome, as it would depend entirely upon the piston kit in question and, specifically, the barrel, but lightweight barrels with gas pistons tend more towards the 2 MOA range in general. Long story short, to get where you want, you'll either have to increase the weight or accept larger groups than what you might want. I would advise against using a 308 heavy barrel unless you're planning on this being a dedicated DMR style build that you'll always be shooting from a supported position. With the weight of the piston system combined with the weight of a heavy barrel I think you'll be looking at ten pounds or more, with most of the weight being out front. I know for me I couldn't comfortably shoot such a gun offhand, especially once optics were added.
If it were me, I would forgo the piston or just go ahead and get the SCAR, or otherwise reconsider the M1A. I think even the M1A is going to put you closer to what you want than a piston operated AR-10. It's also been my experience that DI ARs are just as reliable as any other rifle for the first 200 rounds or so. It's not until after several hundred rounds have been fired that the lubrication starts to become fouled enough to jeopardize reliability. 200 rounds is basically all the average person can carry anyways, so I think the DI vs piston debate is kind of moot. It's also a fact of life that not all AR gas piston systems are created equal, so there's no guarantee that it will be anymore reliable than a DI gun. With modern coatings and lubricants especially, DI is pretty much just as reliable as anything else. It's also worth noting that the good piston kits are very expensive, so it's possible, if not likely, you'll end up with a 12 pound rifle that shoots 1.5 MOA and costs somewhere around 2 grand. There also aren't as many options for aftermarket piston kits for 308.
Part of my rationale for 308 battle rifles is that if you're going to have a caliber that can reach out to 1000 yards, along with the associated weight and recoil, then it behooves the rifle to be accurate enough to actually hit something at that range. 1 MOA at 1000 yards being more or less equal to 10'', let's say for argument's sake that a decent shooter could make consistent center mass hits at around 700-800 yards under average conditions. If you open those groups up to 2 MOA, then at 800 yards you'd be looking at more like 16'' groups under ideal conditions, and that's if the shooter and weather were perfect and there was no margin of error. At 2 MOA, that brings the real world accuracy down to around 500 yards, which is almost within the effective reach of 223. In conclusion, I believe a DI AR-10 is the perfect 308. It's light enough to be handy and accurate enough to justify the large caliber. Of course there are other advantages to 308 like barrier penetration and better terminal performance at intermediate range, so accuracy isn't the
only consideration. But it does make sense to me to squeeze out all you can in all areas, so long as it doesn't involve making any major compromises.
Please excuse the book I just wrote. I'm down with a bug, so I have the time. I hope something in there helps you in your quest.