The M24 light tank also was fitted with this contraption, and the basic load consisted of fourteen smoke and fragmentation rounds. But I cannot imagine how a crew was expected to place a fragmentation round on target!
Similar to, but not as capable as, the smoke grenade dischargers we started tacking onto the M60 series in the 1970s, these were really only for one situation: that stomach-dropping moment when you realize that you have just driven into the enemy's kill zone. The intent is to obscure yourself as a target just long enough for your driver to maximize the potential of the tank's reverse gear and scramble for cover. For that reason, the charges of the rounds for the M3 were reduced, only ranging out to 20-30 yards (meters). The M3 was on the ordnance list of obsolete major items in 1945, which suggests that its installation on tanks was at best a "Hail Mary" play.
It took long enough to build a concealing screen when launching a full array of twelve grenades on more modern tanks. I can't see a single 2" smoke round, launched in haste, being awfully effective.
Allen