I'm sure this does indeed happen. If a criminal element wants to possess or sell something that is in short supply, they will steal it or get it through other means.
I don't think the criminals (the unorganized types) are stockpiling ammo though (north of the border anyways).
Suppose if it got expensive enough, the criminals may have a new niche to fill.
"PSSSSTTTT- hey buddy, you looking for ammo?" (opens trunk of car to show unmarked boxes)
This could happen in particular if taxes were to skyrocket (see cigarettes).