The weapon would need the RIS rail as a valid attachment but it wouldn't need the attachments that are specific to the RIS rail, as an example. My example with the M1 Garand and an RIS rail assumes that the only change you make is to allow the M1 Garand to use the "RIS Rail" object that would be created. The code actually looks at the Attachments.xml and Launchables.xml for the weapon AND all items currently attached to the weapon.

Here's an example that I've been playing with in my test game. I have a bunch of AK-103's. Some of those have grenade launchers attached. Without the GL, the AK-103 can't have a rifle grenade as an attachment (it's not a valid item in either Attachments.xml or Launchables.xml for the AK-103). But as soon as you attach the grenade launcher, you immediately have the ability to attach rifle grenades because the GL itself grants the ability.