I do not understand why you would do this unless it is specific to their setup, but if the tubes are same length and made for 308 just put stock spring and carbine buffer in.

AR 15 tube is shorter by 3/4" or so as I recall in carbine length short(adjustable)

The 308 bolt carrier has to move a bit further than a .223 so a carbine standard .223 tube is too short for the .308 action to move.

Rifle length(fixed) tubes are another topic.

You can read other posts, but the gun should run ok with American brass standard ammo in the 150-180gr range I would think. Then you get into other issues when you deviate.