i'm not a gun expert, but the size of that barrel seems to suggest its some sort of shotgun (ducks and geese)
shotguns are limited - by federal hunting laws - to just 3 "shells" in the magazine at a time. to prevent a rapid fire shooter from emptying a larger "clip" and taking down many birds at the same time.
federal laws also prohibit people from "sawing off" a shotgun to shorten the barrel, and make it more concealable for bank robberies, home invasions, etc. shotguns are lethal at close range, cheap to buy, but ineffective at distance . ..
only police officers may have sawed off shotguns. don't ask me why THEY need them. they aready have 9MM pistols with 15 rounds in the clip, and SWAT sharpshooters with sniper rifles that can take out a perp at hundreds of yards. the purpose of a pump action shotgun in the trunk of the cruiser eludes me - unless its intended as "bait" to be stolen when one of those handcuffed cortonists gets his arms around in front of him and drives off in the cruiser.