Octane and Sirius Black
Sirius had been biding his time -- throwing snowballs at pretty much anyone in his vicinity before laughing and darting off, yes, but also biding his time. Because Remus Lupin was his ultimate target, and Sirius was saving his pummeling for last. The true, real show down. No one would survive it. Probably not even Sirius and Remus.
But between throwing and darting and then throwing some more, Sirius had sort of just stopped paying attention to who he was throwing snowballs at. So when he lobbed one at someone's head it took him a second to realize that he didn't even know that someone. Not at all. Not even a little bit.
"Oi!" He returned after a moment of wide (innocent) eyed staring. The gun was a bit unexpected. Not something he couldn't deal with easily, but it was best not to start meetings with violence, Sirius liked to think. "You're new!"