There is a huge difference between the two calibers.
The .308 is just over .30 inches in diameter and has weights ranging from 110 to 190 grains. It is an old and very versatile round that was designed for military use and is capable of taking most any hunting game in North America..
The .224 Valkyrie is .23 inches in diameter, is much lighter, ranging from 60 - 90 grains and is designed for long distance and was designed entirely for ballistics performance, but not for hunting large game. The .308 cannot be short from an AR15. You need the larger AR10 platform. The .224 Valkyrie is designed specifically for the AR15 platform.

Jean, the 224 Valkyrie even though it's a 22 caliber, uses a longer, thinner profile bullet which increases ballistics dramatically at distance over the 308. 90 and 100 grain bullets staying supersonic longer and having better ballistic coefficient means substantially less drop (over 100 inches at 1k yards and more beyond over the 308). ----------- range365.com/224-valkyrie/

the Valkyrie is lighter and flatter shooting 90 grain bullet at 2700FPS 60 grain @ 3300FPS