I wanted to put some reptilian races in my world, but I'm not sure how to give them the arms needed to hold weapons. From what I can tell, reptiles don't have flexible shoulders like mammals, which is why most of them move in similar ways as opposed to mammals. Is there any way to for reptiles to evolve shoulders and arms with a similar range of movement to animals like primates?
If reptiles evolve sufficient intelligence to create tools, they'd probably evolve similarly flexible shoulders.
Just look at dinosaurs and birds, which are descended from reptiles. The bipedal dinosaurs had a range of motion similar to humans, and birds have a range of motion greater than that of humans.
Likewise, many mammals have a fairly pathetic range of motion in the forelegs, when it comes to sticking the legs out to the side. Ungulates have less motion side-to-side than lizards.
The range of mobility of each joint is something that can evolve rather quickly in response to an actual need for that mobility.