Eh, the way I saw it, the error was kind of an intentional joke. Like, they thought it would be funny if 10K, forever the gun nut, couldn't tell a Barrett 50 cal from an EAD. That's my take anyway.
Oh and here's a possible continuity error for you guys to pour over: when Doc runs over to stop Forman and the other men from killing Murphy, it appears that Cassandra is with him. You would assume that she just ran over from the shooting table. However, when the scene cuts back, it looks like Cass was focused on her shot the whole time, further more, she's not even wearing the same outfit: she's wearing a plaid top, whereas in the previous scene she's has a grey sweatshirt on. This leads me to think that these scenes were shot on different days, and by extention, all the scenes with Murphy are all shot separately (because Murphy's makeup changes every episode to show his worsening condition).
Edit: Aha so it looks like I'm wrong. I watched the rest of the episode and it looks like it was actually a different Asian girl. (they just look all the same don't they)