Maybe I'm just flat-out missing something.

Web Grenades, they have Snare (2). But they have no Blast.

Do you have to hit someone squarely on the face to snare them?

Edit: Apparently they had Blast (4) in Rogue Trader, but Snare worked differently there, so I think I'll go with Blast (2).

In BC Web Grenades do not have Blast, whether that is an oversigth or by design, I do not know.

I do like your idea of giving them Blast (2), as it makes them vicious, but not too vicious.