s400 barrel band
It's not the grub screw, the base of the sleeve is flat (have a look on the Rowan website) and is pushed up to the breechblock. That helps to firm things up so long as the screw is tight. If your worried it will come loose (mine hasn't yet) put a bit of threadlock on it.
Even if you think your on the right road, you'll get run over if you just sit there.