Without stirring up a #### storm this is something that I always wondered about the JR.
They used to be shipped with a Glock magazine which, because it said Glock on it, was perfect for using in the JR as the magazine was not designed for the JR and therefore could hold ten rounds.
Some rifles were shipped with a JR Carbine magazine that was designed/labeled to be used in that semi automatic rifle and they weren't pinned to five rounds.
Just something to be aware of and to double check if you go the JR route.