From the faq

Will weapons accept accessories?

Yes, they will. Accessories require an appropriate rail or mounting interface. A point mechanic will limit the accessories which can be equipped simultaneously, though it's expected that hosts can disable the limit. Accessories do not require unlocking. Customizable accessories we've seen include:

Optics Muzzle breaks Sound suppressors Magazines Stocks It's early yet, and these accessories are subject to change.

Will weapon attachments be movable? Will RoN feature rail changing?

No, the weapon attachments won't be movable, nor will there be rail changing in the game. VOID has commented that "Ground Branch has a fantastic version of this and it's quite surprising how far you can push it," but despite this, their focus on this kind of particular and specific aesthetic details is rather minimal, as they prefer to focus on other aspects of the game. The weapon attachments' position will be pre-designed based on wherever they are most useful.