That's the idle stop screw.lol ...if it's the FX...okay.

You can tighten it more if the barrel still closes. Make use the barral can move freely
It'll vibrate out. Use Blue loctite or CA it. I've double nut it before.
It's there more as s retainer to keep the barrel in place not so much as an idle setting for aircraft models.

For cars it acts as an idle setting...because there's breaks on those things.