Not in MN, but in searching for the legal definition, I found a Riverside CA site that had this definition of "Firearm"
Section 9.12.010 Definition of firearm.
In this chapter, "firearm" means a gun, pistol, rifle, air rifle or air gun, b-b gun, arrow, crossbow, or any other instrument of any kind, character or description which throws or projects a bullet or missile or substance by means of elastic force, air, or explosive substance likely to cause bodily harm. (Ord. 6423 § 1, 1998; prior code § 35.1)
It looks like I've found a place where a squirt gun could be classed as a firearm, depending on how broadly "elastic force" is interpreted.
It's unclear what the phrase "likely to cause bodily harm" means. I don't think a direct-pump squirtgun would qualify in any case, though a pneumatic one like a Super-Soaker® might. On the other hand, a can of oven cleaner would seem to qualify even more, since it throws a substance by means of compressed gas and would be very likely to cause bodily harm.