Call it unorthodox, but I'd make a new bolt-action rifle in .30 caliber an AR-15 Upper Conversion. Basically, a scaled-down UltraMag 50, from Safety Harbor Firearms, but with a few changes...
www.safetyharborfirearms.com
1. I'd thread the barrel extension to take Savage barrels, and headspace using a Brewer Collar. Regular Savage barrels thread on 1.055x20 TPI. Makes for good aftermarket support. The "battle" barrels for this upper would be chrome lined, medium profile, and threaded for flash hiders. Mine would be for 9/16x24 LH (I'm a FAL guy, what can I say). Target versions could take anything available for a Savage.
2. I'd have the oversize side-feed mag well have adaptors that could take BAR or FAL Mags. Thus, you could change the barrel, and shoot 7.62 NATO or 30-06, or their numerous derivative cartridges.
3. A Mauser-style extractor and bolt face would be nice. Positive extraction and feeding is always something you want when operating under gross motor skills situations.
4. Sights would be anything available for ARs, as the flat-top geometry would be the same height as the AR. Front sight would be one of those bolt-on Yankee Hill fold-down towers, or user's choice.
5. Why an AR upper that doesn't feed from the traditional mag well? So our fellow shooters in non-free states can still use their restricted or single-shot AR lowers, and have a mag-fed defense rifle at their disposal.
6. Combine these features with the familiar ergos of the AR lower, and I think you'd have a winner. Add in the aftermarket triggers available for the AR, and that helps, too.
If I couldn't have this, I'd settle for an AIA No.4 Mk.4 in 7.62 NATO, using M-14 Mags. Battle or Target version, I'll take two of each.
Best,
John Bear Ross