Is there any benefit of the extra inch the 12.5 gives over the 11.5 barrel? How much FPS is the extra inch giving me?

I like the 12.5 as it is as short as I can comfortably go to get the mid length gas system.
I can certainly tell it is smoother than a carbine gassed setup.

FPS again/loss is so marginal it's a factor that absolutely doesn't need to be considered.

Gas systems are the only real consideration and IMO, a marginal difference so small it also is irrelevant.

Dropping below 10.3" in 5.56 is where performance and other factors actually kick in.

The real benefit, as mentioned by amphibian, is the 12.5 mid gas, more dwell time, soft shooting, depending on the setup.

Not that the 11.5 can't be as well.

So it's really preference on what you want.

“Soft Shooting” 11.5s and 10.5s can be made with adjustable gas and tuned buffer systems/carriers.
OAL on 12.5 has just never made a lot of sense for me.
Rather step up to my 14.5 SBRs at that point.
But, plenty of guys love their 12.5s.

Probably not a huge difference between the two. These are most people's favorite barrel lengths for 5.56mm. I prefer 11.5" myself since it's shorter. Of course I have a 12.5" in 6.8 SPC and I like that setup a lot also. There's not a huge difference between the two. But the 12.5" suppressed is starting to get a bit long for my tastes.

From another forum:

“velocity tests on a mid length TRIARC 12.5" AR-15 TRACK Barrel 2.0 with NP3 Barrel extension Chambered in 5.56mm
Slr7 adjustable gas block
Brownells lw nitride bcg
Wolf gold 55gr From 2820-2850
Sellier n Bellot 55gr light green box. 2980 to 2890
IMI 55gr 2823-2750
Tula steel case 55gr. 2780-2816
In comparison on the same chronograph
An hk 33 with 16” barrel ran about 50fps faster
Maximum velocity was 3,000 with the green box S&B 55gr”

I have both, suppressed and gas set accordingly. Only difference I can tell is one is 1" longer.

Around 11.5-12.5” is where the velocity curve flattens and velocity gain per inch drops. Both can work with mid-length but the 12.5” is more optimal particularly if you don’t want to mess with adjustable gas. I generally prefer a longer barrel and a shorter can, letting the powder do its work in the bore.  The velocity difference between a 12.5” barrel and an 11.5” barrel can be more significant if one is a “fast” barrel, and one is a “slow” barrel.  But that’s kind of the luck of the draw.

Pick a barrel with the profile and features you like and make it work for whatever length that is.

In some of the comparison tests the velocity differences have been measured up to 200-230 FPS faster with a 12.5vs11.5, which isn't a big deal for paper but you need certain velocities for fragmentation, etc.  I have carbine and midlengths of both lengths and the 12.5" carbine (with reflex suppressor) is my favorite gun.

The difference is velocity and recoil (carbine vs gas) are minor but it's really just personal preference. For me, I went SBR to have the shortest lightest AR that was still effective and reliable so for me, the 12.5 is just too long for what I was trying to achieve.

Personally if I could own only 2 AR's, they"d be an 11.5" SBR and a 14.5" carbine.

There's literally nothing those two guns can't do that I place any real value on. To me, though I see the benefits of 12.5", it falls in to a middle ground I've never really been that drawn to.

Same.  With the 10.5 vs 11.5 being a very tough choice for me.  I love my 10.5s/10.3s..

12.5" will allow you to use a VFG on a pistol build, so there's that. My 11.5 is like 1/8" short of hitting 26" at the muzzle, (flash hider off)

Which one looks cooler? Figure it out and go that route.

This was one of the best resources on the site I think

https://www.ar15.com/forums/ar-15/Suppressed-5-56-SBR-Velocity-Tests-41-Factory-Loads-from-10-5-11-5-12-5-and-14-5-ALL-DONE-/16-714088/

Notice the Black hills TMK.

I went down this road about 15 years ago. I ordered 2 12.5” barrels from Denny’s Guns/Global Tactical, set up one for suppressed use and one for unsuppressed use.  At some point, I acquired an 11.5” barrel but never had a use for it.

My reasoning behind choosing 12.5” over shorter barrels:

-I’m not clearing houses where the most compact gun would be a big advantage.

-12.5” unsuppressed is short enough to get in and out of vehicles easily.  

-the small increase in velocity was desirable because the terrain I play in is wide open desert.

-At the time, 12.5” offered less issues than shorter barrels.

-12.5” barrels should still be less wear on parts than shorter barrels. This is mostly a non issue now as everything is plentiful and I have plenary of replacement parts.

-12.5” barrel + Ops Inc 16th Model works out to around 18” iirc.  Subtract 1” for an 11.5 inch barrel and there was no advantage for me.

-Additional rail length on my newest 12.5” upper has been a benefit too. My first 12.5 upper sport A2 front sights with 7” cheese grater rails.

A properly spec'ed gas port (think <.50 for suppressed) negates the wear variable, but there is a bit of velocity increase with 12.5"

read this it may help to answer your questions, I run 11.5, 14.5, 10.3.

http://www.sadefensejournal.com/wp/barrel-length-studies-in-5-56mm-nato-weapons/

I feel like 12.5 is the current trend just like 11.5 was the trend when 10.3 really took off.

With that aside, I like the 11.5 from DD. Very soft shooter.

I like the extra rail length I can get with a 12.5". With my 10.5" I can't really get a long enough rail to get my support hand out where I want it. I know I could tuck my suppressor but then I need a thicker handguard and I don't like that either. With the 12.5" I can use the SLR 11.7" handguard and get a good support hand position with enough room for a DBAL out front.

I have a 10.3 at work, 11.5 and 12.5 at home.

The 11.5 is a large improvement over the 10.3. Ive shot indoors enough to notice it.

My 11.5 is smooth.

Then I was convinced to try a 12.5....
My 12.5 is smooov. I did not realize the difference until I tried it. (All of the above are carbine)

I have no problem using a 12.5 for clearing buildings. I've been doing this my whole life though so ymmv.

With a suppressor on? Cuz in my experience, that inch or 2 makes a difference.

No cans.

Yep, an 11.5 with a K can is about the same length as a 16" with flash hider.

12.5 master race just to be different (even though it has become fairly trendy).

I've had both, prefer 12.5". It was more pleasant to shoot, which translates to longer range sessions. I could casually plink with the 12.5". The 11.5" I get tired of pretty quick.

I prefer 11.5-14.5" in all platforms

Today I took the 11.5 AR and 12.5 ak105 with me to the range.

There is a member here who did velocity testing with 16, 14.5, 12.5, 11.5 & 10.5.  The velocity difference between 12.5 & 14.5 was negligible, difference between 11.5 & 12.5 was significant.  12.5>11.5 if going with an LPVO & mini can, 10.5>11.5 if going with red dot and full size can, 11.5 is an awkward length IMO, with a full size can it feels a bit long, and a bit too short for a mini can.

12.5 is my favorite. Surprisingly handy with and without suppressor. I run it with a full size can (RECCE 5) and it doesn’t seem unwieldy at all. Barrel profile helps. My Hodge barrel handles noticeably better with suppressor than my DD gov profile. So much so that It seems like a shorter gun.

Not True in multiple tests.  

11.5" is a great length to get velocity and very wieldy

Are we looking at the same data?  Here are 5 of the SD/HD/SHTF/Duty loads that were tested and let’s break the numbers down.

Federal fusion MSR 62gr: 10.5” 2,530, 11.5” 2,611, 12.5” 2,735, 14.5” 2,731
Gold Dot 64gr: 10.5” 2,324, 11.5” 2,360, 12.5” 2515, 14.5” 2,589
Gold Dot 75gr: 10.5” 2,239, 11.5” 2,284, 12.5” 2,391, 14.5” 2,414
Black Hills Sierra 77gr TMK: 10.5” 2,438, 11.5” 2,435, 12.5” 2,586, 14.5” 2,598
Hornady Black 75gr BTHP: 10.5” 2,295, 11.5” 2,278, 12.5” 2,376, 14.5”:2,399

Fusion MSR is my preferred load, 81fps increase from 10.5 to 11.5 but 124fps increase from 11.5 to 12.5 and this test showed a 9fps loss from 12.5 to 14.5 with 2 extra inches.  Seems 12.5” is optimal for running an LPVO.

Gold dot 64gr showed a 36fps increase from 10.5 to 11.5, 55fps from 11.5 to 12.5 and 74fps increase from 12.5 to 14.5.  Obviously 14.5 performed best here, but even still FPS/inch increase from 12.5 to 14.5 was only 37fps, yet you had a 55fps Increase from 11.5 to 12.5 (admittedly not huge with this load, but it was still the largest increase per inch).

Gold dot 75gr showed 45fps gain from 10.5 to 11.5, a significant 107fps going from 11.5 to 12.5 and 23fps going from 12.5 to 14.5 (11.5fps/inch).  Again, another load CLEARLY optimized for a 12.5” barrel.

BH Sierra 77gr showed a 3fps loss going from 10.5-11.5, 151fps gain from 11.5-12.5 and 12fps (6fps) gain going from 12.5-14.5.  Again, another load clearly optimized for 12.5” barrels.

Hornady Black 75gr BTHP showed a 17fps loss going from 10.5-11.5, 98fps gain from 11.5-12.5 and a 23fps gain (11.5fps/inch) going from 12.5-14.5.  Again, another load clearly optimized from 12.5” barrels.

I used to be a fan of the 11.5, then I purchased a Mk18 upper and my affinity for 11.5s went away quickly.  10.3/10.5 feels perfect for running a full size can like a SOCOM RC, it’s still compact and the data shows us almost no real gain from virtually all of the most popular HD/SD/Duty/SHTF loads, in some cases velocity was lost going from 10.5-11.5.

Likewise I used to be a fan of the 14.5 & SOCOM mini combo, but look at the above data, you get very anemic gains in velocity going from 12.5 to 14.5 and the 12.5 with a mini can feels SOOO much handier, it feels handier than an 11.5 with a full size can.

Run what you like, I’ve went through my 11.5/14.5 phase, IMO 10.3/10.5 with full size can and red dot & 12.5 with LPVO is the way to go.  I see the 11.5 as an oddball length, a bit too short to run a mini can, a bit too long to run a full size can.

If those are the loads you're interested in then feel good about your 12.5" choice.  There are multiple ammo types represented in this study, as well as on other pages on the thread, showing negligible increases from 11.5" to 12.5".  

For range ammo I agree, but I’d think that would be a moot point for most of us.

Remember that is only one barrel. A slow 11.5" will really skew the results.

That is certainly a possibility here, although looking at the numbers from the range ammo, my hypothesis would be that the numbers reflected in this testing have more to do with the particular powder chosen in premium vs. range ammo, since the range ammo did in fact show different velocity characteristics.  More testing would be welcome, but with ammo being in short supply, I don’t see that happening any time soon.

I’m really playing devil’s advocate here, I do own and prefer 10.3/10.5 & 12.5 over 11.5 for reasons I’ve already stated.  I didn’t see much love for the 12.5 in this thread, so I wanted to articulate my position in defense of the 12.5.

I keep this in mine.

Others have stated the obvious but didn't see this one.  The extra inch of rail space (depending on your handguard) might give you just a little more room to mount accessories without compromising your grip.  Running into this issue with my 11.3" build and Geissele Fed Rail.  Wish I had more room now that I've added a DBAL A3.  

A legal one depending on lower/buffer tube.  

Pistol vs firearm if on a pistol lower (non SBR).  It is legal (from a Federal standpoint) to run a vertical grip on that 12.5" as they usually break 16 inches.  The 11.5's often come up a a 1/4" short.

I'm splitting the difference with my latest build. Factory Colt 12" barrel from Apex Gun Parts earlier this year, along with Colt parts for most of the rest. It will go on my Colt M4 Commando lower as soon as I get around to assembling it. Stamp is already in hand.

I ran my 10.5 this weekend in a run 'n gun, this is why I'm here looking for info on 12.5" barrels. I just need some more space out there to help me shoot. I usually use a 14.5" gun with 12" rail and it feels great, I'm just looking for as short and as comfortable/stable as I can get.

Do you know what size gas port is recommended for midlength  12.50'' bbl?

My 11.5" only work with cans due to a .045 gas port.  It's softer than any middie 12.5" with a .070 gp.

im doing sbr setup, using the 12.5 upper i built with a criterion core for my "other", im in a ban state... that with my sandman s thats in jail still.. itll be long, but i dont see the gains outweighing the cost to save 1" by going 11.5, im sure ill build one sooner or later just to have one...
what helped me decide on what to build/use for my sbr project:
look at terrain. far shots, short shorts, close neighbor hood sub divisions, house clearing, where you intend to use it, etc.
if your looking to clear houses in the boog, thats how you die, and youll die fast. so thats out.
so realistically, if your engaging across corn fields to battling in woods or desert, longer barrel. if you want a short compact gun for CQB, 10.3, skip the 11.5.
IMO as a civilian, at such close ranges being point blank to 15yds, shot placement will matter more than the fragmentation of the round at a given FPS.
im not saying 11.5 is bad. its great. they all are great. they all have advantages and disadvantages, it comes down to what you want it for specifically.

I have a complete upper from MicroMoa. 12.5 mid gas with 3 position plate for gas port.
.055 suppressed
.073 normal
.082 dirty and / or crap ammo

I run suppressed 100% of the time. No malfunctions with a 556K can.