I am worried 24" will be too long and heavy for what I want.
I underlined "what I want" in your quote because you really need to decide what you actually want. This thread started out looking for a long range rifle, a 1,000yrd AR-15, 24" is not long enough for a proper 1,000yrd AR shooting 223/5.56. 1,000yrd Match Rifles are typically 27" or longer. Both of mine are 29". Back on page 2, you were touting your belief the opinions presented here that 24" vs. 16" barrels didn't really make that much difference.
You took my post the wrong way... I was in agreeance with you that I need a 24" barrel, I was saying that having a barrel 8 inches LONGER as a self defense gun is being emphasized to much. I want a long barrel.
You're shooting yourself in the foot by dragging this foolish "both ends of the spectrum" demand into your thinking. Either you want a 1,000yrd rifle, or you want a defensive rifle. A middle ground rifle won't be great for either. My long range AR's are 29" barrels, my home defense SBR's are 9" and 10.5" barrels. My coyote hunting/knock around AR's are 20" barrels, a bit long and heavy for home defense, and too short to stay supersonic at long range. In this case, "a master of all is a master of none," a 20" will work for anything, but it's not really much better than a 24" for home defense, and it won't be as good at 1,000yrd shooting as the 24".
Will a 20" will stay supersonic to 1000 yards?
JUST BARELY, as I mentioned previously.
With a 20" AR, I can sneak JUST under 1,000yrds before falling subsonic,
With a 20" barrel and a bullet like the 73grn ELD-M or the 77grn Sierra Matchking (HPBT), with BC's in the upper .3's and pushing ~2800-2850 and a 1:7" twist, at my altitude, I can cross 950-1000yrds above 1100fps, and the wheels don't completely fall off by 1,000yrds. At 1,200, it becomes any body's guess.
So far, it seems you're focused on factory ammo, or at least on magazine fed rounds, which also limits your ability to stay supersonic past 1,000yrds. By loading to magazine length, you're limiting your powder capacity and limiting your bullet profile. A 75grn Amax, now called the ELD-M, has a BC of .430, compared to the 73ELDM or 77SMK BC's of ~.360. I can keep a 75 A-max/ELDM supersonic clear out to 1150yrds, but it's seated out to 2.39", whereas the maximum mag length rounds at 2.260" with the 73ELDM and 77SMK fall subsonic at 950ish.
I've seen videos of a few 18"s shooting to 1k.
It's a parlor trick. Guys can edit whatever they want, shoot as many rounds as they want and pretend it's their first round fired, lie about target size, lie about range, you name it... If you look around, you'll find a video of Jerry Miculek shooting a 9mm revolver at 1,000yrds also - are you convinced a 9mm revolver is a good 1,000yrd tool? You'll find a ton of videos from recent years of the "milk jug challenge" where a bunch of people threw hundreds of rounds at a milk jug at 1,000yrds, taking video of every shot one at a time so they could act like it was a 1st round hit. Folks like yourself watching don't realize, a milk jug is a 1/2MOA wide at 1,000yrds, and delivering reliably 1/2MOA at 1,000yrds is something which escapes even the best shooters in the world - so your average slack jaw "Joe Igottarifle" youtuber certainly can't shoot that well.
An 18" AR will fall subsonic somewhere around 850-900, depending upon your elevation.
Also - if a guy puts up a 60" target at 1,000yrds, sure, a guy can hit it with almost any rifle. Give me an hour, a few hundred rounds, and a skilled spotter and I'd bet I could connect with a 22LR pistol on a target that big. If I want to feel good about hitting a target at 1,000yrds, I'm shooting at a 24" target or smaller. Making your target bigger to be able to claim a 1,000yrd hit isn't marksmanship. Throwing rounds at a target until you happen to land one, then posting a video online isn't marksmanship. I've shot my 10.5" AR's at 1,000yrds (which are sub-moa rifles at 100yrds), I can bang the 48" gong more often than I miss, but it's a challenge, and not really reliable. Switch to my 29" match AR's and the 48" gong becomes boring, and the 24" becomes the challenge.
I just feel like 20" will be a better length especially since I want to put a muzzle break on.
As I've said multiple times in this thread - 20" should be your bare minimum if you want to reach 1,000yrds. 24" will buy you additional stability on the bipod, and will buy you another 150fps. A 20" will get you there, but not as easily as a 24".
The muzzle brake isn't really relevant. It adds an inch and a half, and typically weighs about the same as the length of barrel it replaced. Recoil for a 1,000yrd capable AR isn't substantial enough to warrant a brake, and as I mentioned before, brake or no brake, you're not going to be capable of reliably spotting/calling your own shots through a rifle scope with the 223/5.56 at 1,000yrds. With a proper rest and technique, your scope will stay on target with or without the brake, but the dust signature and bullet strikes just aren't big enough from a 223/5.56 to be spotted from a typical 25x or less rifle scope.
Keep in mind - ALL of the above pertains ONLY to bullets with BC's around .360 or higher, and 2800-2850fps or greater. If you do something foolish like throw a 55grn FMJ out there at 1,000yrds, none of the above matters. A 55 FMJ, touting a BC around .240 and leaving home around 3200fps from a 20" barrelwill fall subsonic somewhere around 750yrds, and be an absolute mess at 1,000yrds. If you want to shoot 1,000yrds, feeding from the magazine, there really are only a couple bullets you can shoot. You'll also need a scope with at LEAST 60MOA internal, plus a 20MOA mount.