I am not sure if the barrel will work on a Bighorn. I know that the Bighorn and MK series are interchangeable. IIRC when I tried to swap and LK and MK something didn't work right. I have never shot full bore out of my .54 MK's. It drives tacks with sabots though. I'm not sure what the twist is on an LK in .54cal.

If it was me, I'd leave the Bighorn as is, and search out a complete .54 MK-85 ... you can find a nice used one for very reasonable. I've bought 2 over the years pretty darn cheap, both in excellent shape.