There are agencies that run 10.5s suppressed/unsupressed just fine with 62gr.
It's true. We do, and I love our Daniel Defense Mk18s. Even our legacy CQBR uppers are "okay" (though they definitely don't shoot as nicely). The DDs are a hoot. The service life is short, but they've been perfectly reliable for me with XM556FBI3T ammo.
With that said, there's really no reason to take one over an 11.5", even if concealing them and working in extremely close confines is the name of the game. The 11.5" offers objective benefits over the 10" guns, and the average police officer is not in situations where that short of a rifle is actually of benefit (unless running a can). The 12.5" is an even better match, given it retains 90% of the velocity of a 20" barrel and the concussion is more akin to a 14.5" than it is the 10" or 11" guns...yet still gives you a very short carbine to maneuver inside buildings or deploy from a vehicle with.
It's the ideal GP carbine length unless you're in the business of mounting underslung grenade launchers or bayonets (literally the reason the 14.5" exists).
"Are you ready? Okay. Let's roll."- Last words of Todd Beamer
I won't try to speak on the LE side of the question, but 10.whatever" guns make everything harder than it has to be. The guns run harder, you have to work harder. The window is smaller for reliability and smooth cycling. They're either running smooth or beating themselves to death.
Step up to a 11.5" and life gets way easier. In fact, I'm not sure there is any other 1" change in barrel length for the AR15 that will have the kind of immediate appreciable difference that jumping from 10.5-11.5" will have. My Colt 6933 uppers and my BCM 11.5" ELW will run a wide variety of ammo, from Wolf to full power 5.56 loads without feeling like they're over or under gassed. They run so well and are such a pleasure to shoot that I find myself not shooting my 16" gun, like ever.
That said, the newer MK18 uppers with the crane spec .070" gas port have a much more smooth and proper feeling cycle, but you still know you're rocking a 10" gun. If I had a specific need to go under 11.5" I'd go with a new MK18.
Not sure if we’re thinking of the same thing, but I remember him saying something similar though it wasn’t in regards to barrel length - it was about handguard length. He was making a point for shorter handguards by saying that it’s far easier to punch just the muzzle and barrel through a windshield than it is a muzzle with a flashlight and fat handguard right behind it. Again, not sure if it’s the same reference or not.
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Lot's of good advice above.
At this point in time I still strongly prefer a 5.56mm AR15 16" mid-length barrel with long FF rail for general purpose use or a 11.5-12.5" barrel with a dedicated suppressor.
If you are with a USG LE agency, then use the excellent Federal 62 gr TBBC load off the FBI contract. With a municipal, county, or state LE agency then the 62 or 75 gr Gold Dot are usually much more affordable and work adequately. TSX and GMX are also acceptable options.
Facts matter...Feelings Can Lie
this may be a "gamer" or training thing, but your post made me think of running the 9-hole wall with a friend. He had a 16" barrel with a 15" handguard, and I had a 16" barrel with a ~10" handguard. he had a much harder time "finding the hole" than I did. I realize there's a lot of folks that would say to never poke the barrel beyond "cover", and again I'm not a cop so I can't say how often that might happen in that line of work, but it's definitely a thing (just not sure if it's a thing that matters to anyone).
It was in regards to his preferences for a GP carbine, barrel length, ballistics capability and hand guard length if I remember correctly. In order to have adequate handguard length on a SBR like 10.5 you need as much handguard as you can get making it impossible to jam the muzzle past a windshield due to rake unless there is a suppressor attached to the end of it and f#$% firing a 10.5" from inside a car with no suppressor.
Put me in the 11.5” camp. Even with a vetted manufacturer, I see no reason or benefit to go shorter than 11.5 with 5.56 rifles. The drawbacks already mentioned in this thread and others are not worth it for a rifle that’s an inch shorter in my opinion.