I dunno. Got mixed feelings on the subject. Anybody who throws out any hard and fast rule in this regard is not being genuine with themselves and have likely fell prey to "watching too much tv". Or at the very least the specific need or circumstance during which you would use these firearms have not been articulated well.
First off I would eliminate MY "perceived use" because I don't think that is what we are discussing here.
I live in a rural setting. I rarely go to "the big city" (Raleigh, Durham, Greensboro etc.).
I MIGHT end up in one of those cities 3 or 4 times per year max.
In the spirit of my "perceived use" a lever .30-30, SKS, 16 inch M-4 type rifle, scoped .22 Magnum, or even a shotgun at times will do.
Pistol caliber carbines nor short barreled rifles have any place in my EDC truck/trunk rotation.
But once again, I'm pretty sure that is not what we are discussing.
I "THINK" we are discussing needing more firepower in town because we are in either 1 of 4 situations. I may be overlooking a scenario and am open to hearing what that is.
1. You are in your vehicle when a pretty bad riot situation starts and it's not unreasonable to think you may die at the hands of the masses. We can also include the "Argentina slow side a la FERFAL" scenario here. Venezuela, or whatever.
2. You happen to be in your vehicle in a city the moment the long term SHTF/zombie apocalypse starts.
3. The SHTF/zombie apocalypse has ALREADY started and you need to use a vehicle to get from point A to point B for whatever reason (evacuation, picking up a family member in town, etc.)
4. SHTF starts while you are in vehicle and you must walk to home or safer location.
In the first scenario, you are probably not gonna have passengers firing from your vehicle at other vehicles. Hard target penetration is probably not gonna be a necessary criteria to meet. And you are still likely to face legal scrutiny from the system once the dust settles. In this first scenario it is more than likely a "numbers game" with soft targets and will necessarily be at closer ranges. For me, neither a PCC or an SBR make any sense unless you abandon your vehicle. If you are alone and must fire into soft targets while driving anything other than a handgun is virtually nonsensical.
In scenarios 2 and 3, hard target penetration could very well be a factor, but overwhelmingly from closer ranges, especially if alone in your vehicle.
In scenario 4, I could certainly see penetration on hard targets from a longer distance being desirable. For example, coming up on a true "bottleneck" where an ambush is already established. Few "true" bottlenecks exist around here for folks on foot, but they are certainly more prevalent in an urban environment.
Now I'll try to "qualify" my opinion before I give it.
I think for most folks, calling me a "redneck" is not too big of a leap. I'm down with it. As long as you aren't an "ignorant redneck" "unique" non-range learning opportunities abound! LOL! My lifestyle, coupled with my experience in Fire and EMS, and my time working in a hospital, and oddly enough, my time working in a salvage yard, coupled with the folks I hang out with, have steered me towards compiling certain data that influence my choices. Much of it is anecdotal but repeatable. Much of it shall NEVER be discussed on a forum if for no other reason than avoidance of even the whiff of impropriety or the mere appearance of condoning unsafe behavior. That is what the firebowl nights are for. Kinda.
Vehicles
I've shot whole vehicles or parts of vehicles (think grabbing a door from my neighbors salvage yard and carrying it back to our berm).
I've shot cars and trucks with 9mm from handguns and an MP5 and an MP5SD.
I've shot cars and trucks with 5.56 from 4 inch barrels, 10.5" barrels, 14.5, 16, and 20 inch as well.
I've done it with 7.62X39, 7.62x51, 7.62x51 API, .308, .30-06, .280 (yes .280, long story), 7mm Magnum, 5.7 (handgun only), .22 Magnum, .22LR, .30-30. .35 Remington, 12 gauge buckshot, 12 gauge slugs, .45ACP and 10mm (handguns only), 44 magnum, .357 Magnum, and probably a few I am forgetting.
All the above is shooting into the body of the vehicles, not windshields, which we have also done, but more limited on the calibers.
Oddly enough, the only time we set a vehicle on fire by shooting it, was not with the API rounds, or tracers, but with some regular old Core-Lokt from a .30-06. It was kinda funny. The shots were fired into the front passenger door.
We've shot different rounds into trees to see which ones come out the other side.
We've built fake doors and walls out of wood and shot through them.
We've shot every liquid container known to man full and empty.
I once shot 4-6" ice at my feet with and M1-Carbine while myself and two of my friends were standing on the deep end of a pond.
I've shot large (250 gallon) propane tanks to see what would really penetrate them.
I shot a rabbit one time while playing basketball with some friends.
Participated in a contest too see who could "skip" fired rounds off of water the farthest.
And lots of other sometimes stupid stuff that I won't post here.
I've shot various critters (which I know have very different characteristics from humans, but oftentimes, flesh is flesh.) with all kinds of calibers.
Nothing will burst your academic "ballistics bubble" like shooting a wounded deer in the head with a .357 Magnum with premium ammo from 3 feet away and needing to do it again. Or hitting a snapping turtle in the back with a .44 Mag. and the damn thing still catches and kills a duckling.
I've tried hitting targets from moving vehicles as the driver.
I've tried hitting targets from a moving vehicle as a passenger (as a passenger I have tried it with select fire and semi., handgun and long gun, shotguns with short barrels and 18 inch barrels).
I say all that to preface THIS.
In scenarios 1,2, and 3, if the choice is between a 9mm SBR and a 5.56 SBR, I will take the 9mm every time. And it will be loaded with ball ammo. I'll have 9mm HPs on hand if circumstances change. They will be in the mags of my handgun. The 5.56 SBR does not offer me enough advantage to put up with the disadvantages (those being obnoxious noise and blast, and less penetration on hard targets from close to intermediate range). Even if they are both suppressed and select fire I still choose 9mm. If money was no object my ideal truck gun for the first 3 scenarios I outlined would be an MP-5SD. Since money is a factor, I would most likely carry my 9mm-AR with 10.5" barrel or my Glock 17 Micro-Roni setup. But I promise you they would not be used unless I had a passenger or I exit the vehicle.
I'll admit, scenario 4 gives me pause with unilaterally saying I would go with 9mm carbine. If the real deal SHTF started while I was in a city, I would hope like hell I had a 16 inch AR or full sized AK.
So, I'm torn.
For me, if I am carrying something in addition to my two handguns I carry every day, In the city, it's either gonna be a 16" AR in 5.56, or a short 9mm.
I have a 10.5" AR in 5.56. I hate to seem like I am really down on them. I'm not. I enjoy mine.
I live in the country. The place I work is not in the city. I have a few guns to choose from. I don't carry a long gun on a regular basis nor do I plan to at this time. If I did, it would probably be a scoped .30-30. It matches "my outfit". LOL! And my eyesight seems to be changing every couple of months here lately.
I don't live in or frequent the big city. If I did, and I decided to keep a long gun in the vehicle, and it needed to be "short", it would be a 9mm. I don't think the extra "umph" in a short barreled 5.56 means anything to me. It certainly doesn't translate to better penetration on hard targets in my experience. Ball vs. ball anyway. Or even green tip vs. ball.
Most of this fun and sometimes stupid stuff I and my friends have done over the years was of course prior to the "benefit" of phones with cameras. Probably luckily so in some cases. Those of you who know me will take me at my word, those that don't may not. It's okay. It's a gorgeous day and I am not mad or bored enough to worry about "winning the nets" today.
But on a related note, I did receive several hundred rounds of the "new" M855-A1 (Alpha One) 5.56 rounds for my birthday.I haven't shot any yet. Maybe my neighbor has a couple of cars I can shoot up. If I do I'll shoot some comparisons and post the results. Maybe I should take a fire extinguisher this time. I'm not sure how tungsten plays with insulation in door panels yet.