I know the FN FS2000 rifles used a compression washer that sits within the FH and contacts the muzzle end, not a shoulder on the barrel. I haven't looked inside the surefire muzzle device lately but I do believe they have a flat surface that might work with the FN crush washer. If you are running a can, maybe the crush washer isn't a good idea. Custom shims might be a better option for alignment.
What is the problem with the shoulder on the warcomp? does it bottom out on the splines?