I have running an AR308 for 5 years now. The breakdown:
Dpms Oracle 308 Upper, lower, and BCG. No alterations. Criterion m118 20" barrel. AR308 Rifle length gas system. Vltor A5 stock extension(using this extention allows me to use standard length 3.25 inch AR15 type buffers. The Vltor stock extension is 7.75 inchs long. As a side note, there are a couple of other makes of stock extensions that are also 7.75 in long. Choose wisely. Note: the standard AR15 stock extension is 7.0 inchs long. If you want to run a Vltor A5 buffer with an AR308, you are out of luck. The AR308 BCG is about .75 inches longer than the AR15 BCG. The Vltor A5 buffer is 4.0 inchs long. It is .75 inchs too long to work inside an AR308 upper receiver because the AR308 BCG is longer than an AR15 receiver. Are you seeing the math here? Note that the Vlto A5 buffer was developed for the Marines as a way for them to use an adjustable stock on their 20" barreled M16 and reduce a bit more recoil. The Vltor A5 system works well for use in what it was intended for. You can also it on any AR15 to reduce recoil and slow down the cyclic rate making it easier to shoot. Yes this is confusing. I went through this awhile back. Post back and I will check in for q.