What Carbine Adjustable Buffer ?
With a AR15 carbine length buffer tube, and a LFAR BCG, you will need a 2.5" buffer... KAK makes a 5.3oz 2.5" 308 buffer ( I think it is sold by various other places, under the brand Midway )
That said 2 of my LFAR's are setup that way, they also use the 2.5"...