I think Mythbusters did a test on this and found that anything smaller than a .50 was unable to knock their test dummy off the rig (and even that was barely able to). A shotgun managed to do it, but they had to be fairly close so that all the pellets hit the target.
To support your flailing argument, there's an old video about of a disgruntled former client attacking a lawyer on camera with a small calibre revolver (I think a .38). The lawyer managed to keep a telegraph pole between him and his attacker and walked away from the incident after the client ran out of ammo, even though he had been shot several times (presumably in non-fatal areas).