I also run a Huntertown Guardian on mine. It has ate about 90% of the brands that I have ran through it. It didn't care for some of the older black box Winchester target ammo.
The gun definitely seems to get dirtier with the can on. I have not noticed a difference with mine with and without the can on.
The 60gr subsonics ran surprisingly well in my M&P22 and seemed to hit the steel plates harder than a lot of the super sonic 36gr federal bulk rounds. The twist rate in that M&P barrel must be just right for those 60gr rounds because they were awesome. In a rifle the same ammo wasn't accurate a foot away from the barrel. I don't remember the brand of the guys can that was used.