I talked to Woody Woodal at LOTHAR WALTHER about replacement barrels for KNIGHTS ARMAMENT (KAC) SR-25 and variants.
All you have to do is send in your old barrel/extension and bolt to LOTHAR WALTHER and they will copy the contour of the original barrel in your choice of carbon or stainless steel. The carbon steel would need to be phosphated or parkerized where applicable. They will remove your barrel extension and headspace your bolt/barrel extension/barrel assembly.
I have close to 1 dozen LOTHAR WALTHER barrels on my rifles and would highly recommend them.