I am thinking about making an AR Pistol.  What barrel length should I go with and why?

7.5, 10.3, 10.5

I'd stick with the 10.5. With a h-2 buffer..  Keep it simple.

10.5-12 for 223. Because it suffers greatly from a lack of barrel length.
9-12 for 300 blkout. Because it does not suffer quite as much.
They both loose ballisticly with anything under 18"ish...how much you want to sacrifice is up to you ?
Depending the task at hand.. At some point it is just pointless to have a barrel so short you throw away any good  ballistics still  available.

Your choice may depend on your use


My first was 7 1/2"   it shoots a fine fireball, everyone who has shot it, loves it...
My second  11 1/2" from what I've read, function and ballistic wise, better than the smaller barrels
When I built the 11 1/2" I wanted to keep it as compact as possible without flash hider I'm just a hair under 26"

For what its worth, both function perfectly

I went form an 11.5 to a 10.5 to a 10.3

I suggest you go 10.3.

Is this going to be a range toy or something you plan to actually use, maybe suppress?

223/5.56mm at less than 10" is a range toy.

I dig the 11.5" as it seems to be a good compromise between compactness, reliability, durability, and velocity.

< 10.5" is kind of a toy.

10.5" can be a hair tricky to get dialed in, but if you're going short, you may as well go short.(within reason)

12"-12.5" and you're getting close to 14.5" pinned.

5.56?  I went 10.5" for suppressor host.  12.5" for some bad ass to be SBR

you can get 19" of penetration & full expansion with .223 defensive rounds out of 7.5", the only reason not to go that short is if you plan to suppress as most companies won't warranty under 10".

The first short barreled AR was the XM177 and it was manufactured by Colt - who had plenty of experience and insight to the original design. If it could have been a more effective combat weapon shorter, then it's likely they would have - but didn't. 10.5" was considered the shortest effective barrel length to get the needed ballistics from 5.56 for that application - shooting at targets likely from contact distance to over 100m. Not to forget they eventually added a sound moderator as users weren't wearing hearing protection due to the situations.

Now, those similar weapons have silencers and most of the users are equipped with sound enhancing comms in protective head gear.

For the individual builder it's a question of how far you intend to shoot it and at what target. Range, Target. That determines what amount of power you need and what the specific effect is desired. Short range live targets are different than long range paper targets. You build the gun to match the requirement. Any deviation is either for a specific ballistic advantage - or is a range toy for flames and noise.

What is your Range and Target?

FWIW, The original XM177 E1 had a 10" barrel, not a 10.5"...  But I agree. The 11.5" carbines are very reliable - the original XM177 suffered from bolt bounce - something they cured apparently by putting an A1 stock and buffer on the. (Heavier buffer cure.) Shorties are very overgassed. I would go with Mk18 gas port specs if possible.

I like my DD 10.3, good overall length and don't give up too much in ballistics.

My next build will be a 300BO in 8.5 length.

Go with whatever feels best to you.
Any length is reliable and has plenty of power to drop the same target as a longer barrel can. At 1000 yards would they be an ideal gun, no. At 300 and less yards where all shooting takes place, yes.
Short barrel AR's are not ''range toys'', I hate the goofy term. They are the same weapon with the same ability as longer AR's in most roles but in a short package.
Let their be no mistake, a SBR / AR pistol is not a toy, it is an effective weapon.

I have a 7.5'' and an 11.5'' and I like both. They both are similar to shoot and are both 100% reliable with all ammo. I would trust my life to either one

Just like blonds are more fun, so are short barrels. That said I went with a 10.5" on the 223 and 8.5" on 300blk. The 7.62x39 has the 7.5". Make sure you get a linear brake or flash diverter for it.

I like 11.5

5.5"

Why? Because 9mm is a hoot to shoot

I chose a 10.5" from Andro Corp for my 5.56 pistol.
One of the reasons was after reading a very detailed article on short barrel performance.
You can find it here:
http://www.sadefensejournal.com/wp/?p=1093

I went 7.5" becuase I'm making a pistol so I wanted it short.

About 8 years later it still runs flawlessly. With my 45gr TSX load I get full expansion and 13" penatration in calibrated gelatin from 15yards.

8"

Give you enough clearance to mount a suppressor with most 7" rails.

If we're talking 9mm/40 S&W/ 45 ACP go shorter and hide the can under the rail.  3.5"-5"

Vs the .45ACP -


And just for fun, the vaunted 10mm muzzle energy by barrel length.



Seems like a nice 7-10" pistol barrel with .223 is less of a "toy".

best

mqqn

There's "experts" everywhere on AR15.com blasting the 7.5" as nothing more than a "range toy" with "ice pick" wounding effects of a .22LR. #1. Stop regurgitating everything you read on firearm forums. #2 I don't see any of you range/forum warriors lining up to take a shot to the arm to demonstrate the "ice pick" wound you allege that it leaves. #3. Entirely too much emphasis is put on fragmentation of the 5.56. Even from a full length rifle, fragmentation isn't a guarantee... it happens sometimes, but not all the time. There are so many factors other than velocity that contribute to whether a round fragments. And no enemy that you could ever possibly need to engage will be made of ballistic gel. That said, I'll include a video link to M855 fired from a 7.5" into ballistic gel (at 2234 fps terminal velocity... I believe @ 15 yards) at the end of this post to disprove this nonsense that it's ineffective. It does a hell of a lot more damage than some of you think it does. They definitely don't sell "ice picks" that large. The US Secret Service uses the FN P90. The 5.7x28 has nearly the exact same muzzle velocity as the M193 or M855 does out of a 7.5" with half the bullet weight and a third of the energy delivered. Why on earth would we protect POTUS with range toys? We aren't. Neither the FN P90 and a 7.5" AR pistol or SBR are range toys, and both will get the CQC job done. By the way, if you compare the 7.5" to a .22LR, you need to smack yourself in the face with a frying pan and hopefully wake up smarter. And lastly, watch the M855 ballistics test in the following link. There is no way anyone can tell me that a double-tap to the chest from a 7.5" wouldn't neutralize a target. I've dropped coyotes at considerable distance with a single shot from my 7.5" and I'm confident that I can neutralize any potential enemy with it as well. I trust it just as much as a defense weapon as I do my 14.5" Bushmaster. Watch this video: https://www.youtube.com/watch?v=95sQ4fs7mwg

 
What he said. 100%

I like my 8.5.

I figure 8.5 is a compromise between The 7 and 7.5 and the 10.5.

I used a PSA 8.5" on a recent build and it works great.  Bigger fire ball compared to my 10.5" but it is just that much more fun.  Still very accurate

I have a 7", 10.3", and 11.5".  All run great. My 7" is fun, and it hits steel accurately and pretty hard at 100 yards. The other two are great also. I migrate to the 10.3" most often, though so that is why it has my vote. But in pure ARFCOM spirit, "get both".

7.5" because  it's short.  Know your range limitations and you'll do fine.  My so called "range toy" makes a perfect sbr/pdw.  Perfect for room clearing social distances.  Accurate out to 150 yards.  Vmax bullets are phenomenal on rodents and varmint of the four and two legged kind.  KX3 to tame concussion and blast to the shootee.

Kx3  blast baffle will help slow the action down by keeping some gases to slow the dwell time.  Oring on the extractor, h2  or similar heavy buffer.  Mine works fine with full auto bcg, enidine  buffer, oring.  I replaced the enidine  buffer with a Spikes ST2.  No problems.  Runs like a top.  Very compact, which is what I wanted.