Unless you have a standard A2 buffer tube, the AR-15 uses a <span style="font-style: italic">CARBINE</span> length buffer, spring, and tube. What you will need for the Magpul PRS is a <span style="font-style: italic">RIFLE</span> length buffer, spring, and tube.
Make sure when you get the tube it's mil-spec extruded at the end, not welded as the additional recoil impulse from the .308 has the the capability of cracking the end of the buffer tube.