I don't know what model the K3B is, but I do have 3 oly/stg guns. I've swapped out two of the uppers I had for flat tops. It takes about a half hour including the 15 minutes to find all the tools. I didn't have any problems swaping the A2 for the flat top. All of the manufacturers have some combinations of uppers and lowers that fit tight or loose and thats about the only thing you'll run into. The gun with the most slop in the upper to lower fit that I've had was a DMPS. The olympics I have seem to be on the tighter side.