(1) Stand in for an SBR. It can be a substitute for a short barreled rifle (SBR), although, in some people's minds, a poor substitution at that. With the pistol, you get the short barrel but without having to pay the $200 tax and waiting an indeterminate time for your SBR application be processed by the ATF. Those that favor the pistol over the SBR also point to the fact that there is no special paperwork to transport an AR pistol across state lines, states that don't allow a loaded rifle to be carried in automobile generally allow a pistol, and, given that you are allowed to shoulder arm braces as in impromptu stock, there is no real reason to go through the SBR process. But if you want an SBR, it allows you to at least enjoy your erstwhile SBR in a neutered form until you get your approval from the ATF, and it is easy to make the final conversion just by adding the stock.
The foregoing still skirts the question, though, of why have a short barreled rifle-caliber weapon in the first place.
(2) Home defense. One of the primary advantages to a short-barreled weapon is maneuverability in tight spaces, such as your home. If you have it set up with a cheek rest of some sort or a shoulderable arm brace, you can keep the weapon at ready while still squeezing through doors and around corners. It may not be the manstopper that is the shotgun, and it may have much greater concussion and flash than a handgun, but has better maneuverability than the shotgun and, depending on your ammo choice, better terminal ballistics than a handgun round.
(3) Trunk gun. What is a "trunk gun" probably depends on who you ask. One author describes the "trunk gun" as "a sturdy, reliable, and not-too-expensive firearm that can be kept tucked away in a car or boat for plinking, hunting and, in a pinch, self-defense." Another, taking more of a prepper stance, sees a trunk gun as a weapon that "can be used in a survival scenario where you have to leave your vehicle behind and huff it back on foot." While, for the tactical crowd, "the two most important attributes of a Trunk Gun is that it’s dedicated to a vehicle and that it offers an increased capability over your sidearm." But yet another author cautions:
The trunk gun is not there to go and get, and then re-enter a situation to deal with the threat ourselves. If you have made it to your car, you can safely exit the area and that is our smartest plan. If you encounter jihad on your way to making an escape, by all means burn them down, but most of these events are over in moments and by the time you get to your car, get your big gun and get back to the fight, it will be over and you will be a target for the responding officers. We, the armed, non-sworn, civilians of this country, do not carry guns to be Junior G-Men. We do not have belt pistols and trunk guns to seek out and engage terrorists. That is the job of the professionals, and doing so may well see us get shot by police instead of terrorists. Shot is shot, and it sucks.
Rather, the trunk gun is there to give the citizen more capability over a handgun to cope with an elevated threat situation in their area. While we’re trying to get out of Dodge in a situation where we have advance warning of heightened danger, a long gun up front gives us more options.I lean toward the self-defense or tactical perspective. Police often have "trunk guns"--rifles in the trunk or trunk lid of a squad car for use at distances where a pistol and shotgun are impractical or better accuracy is required. To me, this latter example probably epitomizes what the self-defense oriented person thinks of when he thinks "trunk gun."
(4) Defense against mobs/gangs. Two words: Alexian Lien. Who? What? On September 29, 2013, Lien and his family, while driving in their SUV, were caught up in a gang of motorcycle hooligans called the Hollywood Stuntz. According to Lien, as the motorcycles crowded around his vehicle, he bumped the back of one in front of his vehicle. This resulted in gang members, including an officer with the NYPD, to beat on the vehicle and yell at Lien. Lien panicked and the chase was on. When Lien made the mistake of stopping, gang members, including the aforementioned police officer, dragged him from the vehicle and beat him (video).
Although Lien's beating may be atypical, it is not uncommon for such gangs to shut down traffic or take over intersections and threaten or assault other drivers. Some stories as examples:
- "Biker doing 'wheelies' crashes, closes Dallas-area freeway"--Spectrum News-Austin (July 16, 2018).
- "Hundreds of Motorcyclists Shut Down I-8 With Tricks: CHP"--NBC San Diego (Jan. 14, 2018).
- "Watch out! Cops warn of swarming dirt bikers and ATV riders on MLK holiday"--Sun Sentinel (Jan. 10, 2018).
- "Motorcyclists swarm California freeway and assault driver, police say"--Mercury News (Jan. 8, 2018).
- "200 motorcycles doing doughnuts, wheelies close I-880; 5 arrested"--Mercury News (Oct. 30, 2017).
- "Motorcycle mob sought for Toronto highway mayhem"--Toronto Sun (Aug. 10, 2017).
- "Police in California search for bikers after violent highway attack captured on video"--CBS News (March 10, 2017).
An AR pistol gives you something that is maneuverable within the vehicle, capable of a large number of shots, and much better ballistics against heavy clothing and/or Kevlar. Obviously, it does no good if it is in your trunk, but it is probably easier to figure out a way to keep an AR pistol handy than a larger weapon.