Wanting a .308 in a AR-15 chassis is good in theory, but the problem is in the chassis.
AR-15 lowers and magazines are designed for smaller caliber cartridges. However, there are 3 calibers that I would suggest you look into if you are wanting to keep your current lower and just swap uppers for long range vs short range.
1. 6.5 Grendel
2. .300 AAC Blackout (.300 Whisper will fire in the Blackout barrel no problem)
3. 6.8 SPC II
The 6.5 Grendel is the superior round of these three when it comes to distances up to 800 yards. There are multiple companies making uppers for this round. Hornady and Alexander Arms make the best ammo. This round takes a special magazine for proper feeding. The round itself is still supersonic at 1000 yards and has a fantastic BC compared to other .30 caliber rounds.
The .300 AAC Blackout was originally invented for suppressed CQB purposes and fitting a .30 caliber bullet into an AR-15 chassis. Most of the ammo out there would be good to 600-700 yards max as they are typically not that fast. This round will work with all steel and aluminum GI style magazines and followers. Magpul PMAGS would need to be modified for proper feeding.
The 6.8 SPC and SPC II have gone through some changes over the years. The II designation is a recent change that has become the new standard for this round. It is also a .30 caliber bullet that was designed for hard hitting CQB purposes, but good for longer ranges as well. This round requires specific magazines for proper feeding.
There are also uppers chambered in 7.62 x 39 Russian (AK-47 round). I myself have a dedicated 20" AR-15 in 7.62x39 that is a tack driver at 100 yards and would make a nice hog gun. But the round is only effective out to 400 yards or so at best. If you go this route, I highly suggest getting your upper and BCG from Model 1 Sales. I worked with them in testing their prototype bolt carrier group for this round. The problem has always been proper ignition of Russian produced ammo in this caliber. The head engineer at Model 1 Sales sent me a prototype BCG back in 2008 when I was having ignition and FTF issues with Russian ammo. The problems were with the firing pin and the bolt, in relation to the pin not hitting the primers deep enough for reliable ignition. The prototype BCG had a revised bolt and firing pin that allowed the firing pin to protrude deeper. Before the swap I was getting FTF constantly. After swapping to the prototype, my gun fired everything I fed it. Both domestic brass ammo and everything foreign. They have since made this BCG standard in all complete uppers they sell now. The 7.62x39 caliber uppers require caliber specific magazines as well.
C-Products used to make the best magazines for these 3 rounds, but they have since gone out of business. Now, Armalite makes them for the 6.5 Grendel, 6.8 SPC and the 7.62x39 rounds.
The .308 WIN will only fit into an AR-10 chassis rifle.
However, Colt recently released a new rifle that can use both a 5.56 Colt MIL Spec upper and their .308 upper. It is called the LE901-16S. It is a new modular design that uses an insert that is machined into the 5.56 upper that accepts your standard 5.56/.223 magazine when attached. It shares the same outside dimensions as a standard AR-10 chassis rifle.
http://www.coltsmfg.com/Catalog/ColtRifles/ColtLE90116S.aspx
But other than this new rifle, standard AR-15 lowers can not run the .308 WIN cartridge.
Hope this helps.
Patrick
