New (crack?) theory: Gun Guy is Maurice Cole. The boy's unhinged AND has a grudge against Ciel. Possibly Soma as well, since he wasn't exactly being subtle when he was tailing Maurice at Weston.
Yep, I’ve discussed this idea before (look here). Maurice would indeed have a motive for wanting to harm Soma and Ciel. And Soma would recognize him. I think this attack is connected to Bravat’s sect but Maurice may actually be involved in his (just like with the former P4). So there’s probably still a possibility that it’s actually Maurice.
However, since I believe that the attacker with the knives is Lord Polaris there’s a good chance that the one with the gun is Sirius. And I don’t think Maurice is Sirius. We’ll see if we get more information on that person in the next chapter from Soma.