The 40mm buckshot rounds will indeed clear an alley, though the WP round does pretty well at night too.
Unlike the other handguns so far named in the thread, the chopped M79s also have the ability to put the hurt on a light armored vehicle, using a M430 or M433 40mm HEDP round with their shaped charge warhead.
There's also the interesting development of the XM1822 40mm shell adapter for the M79 and M203, which launch 18 rounds of .22 ammo from an M79 shell case...that too should be handy for alley work. And they're reloadable....