Made it to the range today with the KAK heavyweight buffer and a Tubbs 42 coil flat spring in the pistol. I brought a regular .308 spring, a regular carbine spring, Sprinco red and blue springs, steel and aluminum weights, and tools. I was ready to try as a many combinations as it took to get the optimal balance of spring resistance and weight.

Turns out the KAK heavyweight buffer and Tubbs spring worked really well. The recoil was mild but the bolt locked open and 147 grain loads. The 124 grain loads still burned the sides of the cases a little bit the recoil was reduced.

At some point I will chrono loads with the different springs but this seems to be a good combination.