Well actually a canister shot with teflon lining on the inside would do, but should we delve into the pseudo science governed fantasy, I think if the problem is which material and what thickness to make the shell from to:

a) be safe to handle
b) survive the firing
c) Crack on impact of a relatively soft target

The only thing I have is using low velocity shell fired from a smoothbore gun, and most grimm will simply dodge that >_>

Seriously, it would be simpler to mount a bloody laser xD Penny displayed quite well that a rather powerfull thing can be mounted in a small frame.