The spell is good--not that he'd admit that, even to himself--to himself he only thinks how fucking typical it is, that Snape would knock off a spell like that at him, underhanded, sneaky, typical--and he barely has time to get his wand up to do any sort of deflection. He gets up a shield charm, but only just--and the stunning spell still hits him, enough that his arm feels numb with it.

But it's not his wand arm, at least--he's got command of that, and he flicks his wand toward Snape, quickly--jelly-legs jinx, rudimentary and childish and it doesn't matter, anything to throw him off before he's got time to take advantage of the hit he got in on Sirius.