(cross-posted from ar15 - ar15 forum)

Title says it all --

I love the look that the 10.5s & dissipators (11.5 slightly too long) have -- FH just forward of the FSB -- and wondered if anyone had considered doing that with a 12.5-13" barrel and midlength 9 inch handguard?

I think it would look great and would eliminate a couple of problems:
1. 10.5s are REALLY loud (although that can be fun )
2. A lot of velocity loss with 10.5
3. Many silencer mfrs will not warranty their cans with anything shorter than 11.5

Anyone seen or heard of anyone doing an upper like this? I'm broke-ish otherwise I'd try it real soon. Maybe later this year I will build one.

Noveske Rifle Works offers a 14 1/2" mid length.

There are two different questions or assumptions here...

If you want a mid-length handguard on a 13" barrel, that's no problem.
I would put a gas block at the carbine location and then a mid-length handguard that covers it.

If you want a 13" barrel with a mid-length gas system, now you are looking for trouble.

10.5" barrels with carbine-length gas systems are not known for their reliability with as wide a range of ammo as a similar 14.5" barrel would be reliable with.
This is due to the proximity of the gas port to the muzzle.
You need a certain amount of "dwell time" (for lack of a better term) where the gas enters the gas system while the bullet is still in the bore.

The mid-length specs basically call for moving the gas port 2" further forward.
That gets you the same gas system issues with a 13" mid-length that you have with an 11" carbine-length gas system barrel.

Typical 14.5" barrels with carbine gas systems have 6.7" of bore in front of the gas port.
20" rifle systems have 6.8"
16" barrels with carbine gas systems have 8.2" and mid-length 16" barrels would have 6.2"
It's a general guideline that you do not want to go much under 5" before reliability starts to suffer.
11.5" barrels with carbine gas systems have 3.7" in front of the gas port (seeing a pattern here yet?)
10.5" has 2.7" in front of the barrel.
The 14.5" mid-length barrel mentioned in another post above would have 4.7" which is about as short as you want to go.
Lastly, your theoretical 13" barrel with a carbine-length gas system and a mid-length handguard would have 5.2" and you are looking good.
Move that gas port out to mid-length and you only have 3.2", much less than optimum, but it could still be made to run.

Randall Rausch
www.ar15barrels.com

I've heard that the military has had some success with 12.5 middies. YMMV.

Dwell time isn't the only issue--a middie 13" would have a lesser peak presure, which is better.

Quoted: Dwell time isn't the only issue--a middie 13" would have a lesser peak presure, which is better.

I believe that you mean "port pressure", not "peak pressure".

The peak pressure happens in the first few inches of bullet travel.
Port pressure is what is unleashed into the gas system.
Port diameter is used to meter the gas so pressure is actually not as important as total volume that the system gets fed.
Having said that, the mid-length gas system is more gentle and the rifle system is even kinder to the action.
I still think that the 13" barrel and mid-length gas system is a step backwards in total reliability/versatility.

Randall Rausch
www.ar15barrels.com

I meant peak port pressure. I like the flat, wide pressure curve of the midlength.

If I set up a barrel like this, I would use a KFH or a can to increase backpressure, just to be safe.

Quoted: If I set up a barrel like this, I would use a KFH or a can to increase backpressure, just to be safe.

And that would be a very good way to go...
The can does add at least an inch of effective barrel length as far as reliability is concerned.

Randall