I want one too! I have a Daisy 990 dual-power rifle, can use CO2 or pump it manually. The more I pump it beyond the regular 8 to 10 pumps gives me greater range. But no way would I get 350 yards! It can be deadly at close range if mishandled, I've put BBs through several boards and sheetrock which is thicker than my skull. But it ain't a firearm.
“But it ain’t a firearm.”
That’s wishful thinking. Any armament whose purpose is to “to emit or let fly an object” is defined by the dictionaries as the “firing” of an armament, whether or not any combustion is involved. You can fire a rock from a sling, catapult, or ballistae as they did in the ancient wars as a form of artillery.