Keep the steel wool away from stainless or the problem is likely to come back with a vengeance.
The carbon steel in the wool will smear on the surface and cause even more rust.
If the rust is discrete small spots on a new gun it is likely from the steel tooling used during manufacture.
Passivation treatments can help, but sometimes spots still get through.
Scotch-brite is better f you need to use an abrasive. You can restore brushed stainless by using Scotch-brite and restoring the scratch pattern.
For bead blasted try CLP or even liquid wrench. They can remove rust without doing much harm to good metal. The only real way to touch up bead blasting is with more blasting. If you have blasting touched up be sure the media is new or has not been used on carbon steel. Like steel wool, blasting media contaminated with carbon steel can leave enough metal behind to cause further surface rust to appear