For the 308AR, there are two carbine receiver extensions.
You can use an AR15 Carbine receiver extension, and you must use 308AR Carbine buffers - little short fukkers. That receiver extension has a 7" internal depth.
You can use the Armalite AR10 Carbine receiver extension, which runs a standard-length AR15 H3 Carbine buffer. That receiver extension has an internal depth of 7 5/8".
Now, VLTOR came up with the A5 system to give US Marines a collapsible stock system to use on their M16 rifles, with their 20" barrels, WHILE maintaining the reliability of running the rifle-length recoil system. It uses (basically) the Armalite AR10 Carbine receiver extension, but with a special, longer, "carbine" buffer that will run in the AR-15.
You cannot use the VLTOR A5 buffer.
However, if you run the Armalite AR10 Carbine receiver extension and the typical AR15 H3 Carbine buffer, you have essentially created the same thing, for the 308AR. Use the Armalite EA1095 buffer spring.
You cannot buy the VLTOR A5 System, complete, and bolt it up to a 308AR and get it to run.