My understanding (with caveat) is that the pistol length is beneficial for subsonic 300BLK with suppressor but since those are illegal in Hawaii, what's the use? For a supersonic round, is there much difference in performance between a pistol length and carbine length in terms of accuracy and functionality?

My understanding (with caveat) is that the pistol length is beneficial for subsonic 300BLK with suppressor but since those are illegal in Hawaii, what's the use? For a supersonic round, is there much difference in performance between a pistol length and carbine length in terms of accuracy and functionality?

It depends on the gas port hole drilled into the barrel.

A larger gas port may require;  a longer gas travel to allow the bolt to cycle without a jam.
A smaller gas port may require ; a longer gas travel to delay the bolt from cycling to fast.


The length of the tube has to do with the cycle rate you are trying to achieve under full auto. 800rd/min 900rd/min.
under semi it doesn't really matter.

If you are just using different ammo, you may just get an adjustable gas block.

I would buy what you're going to shoot.  Like you stated above, when you had the mid length but shot cheap ammo, you had malfunctions.  So if you're going to continue to shoot the cheap stuff, buy a carbine length.  My carbine length has shoot wolf ammo with no malfunctions yet.

The length of the gas system (length from the receiver to the gas port) should increase as the barrel length increases.
The reason has to do with “dwell time” – the length of time that the bullet is in the barrel after the shot is fired.

On a longer barrel, the dwell time will be longer, because the bullet has to travel a longer distance. Specifically, there is a
tiny amount of time when the bullet is traveling through the barrel and it is past the gas port, but it’s still in the barrel. During
this time, there is hot gas traveling through the gas tube into the receiver. As soon as the bullet leaves the muzzle, the gas
stops flowing.

If there is too much barrel length after the gas port, then too much gas flows into the receiver and it can causes issues with
excessive recoil and wear on the rifle.

If there is not enough barrel length after the gas port, then too little gas flows into the receiver and the rifle may not cycle or
may jam.

System	Barrel Length	Port Distance
Pistol	Less than 10 inches	4 inches
Carbine	10-18 inches	7 inches
Mid	14-20 inches	9 inches
Rifle	20 plus inches	12 inches

SNIP

I mostly buy and shoot decent ammo.  I was really looking into Tula during when ammo availability and price went crazy in the 2012-2013 time frame.  Tula was still cheap then, but now it has been hard to find and when I have found it, it was the same price as 193.  I'm not going to spend $300-350/1000 for Tula! 

Many people say all of your guns should (reliably) shoot all the ammo you have.  I haven't really tried to tweak my BCM and LMT to get to run well on Tula as they have ran fine on decent ammo.  Yeah, maybe the carbine length guns are the "answer" for me for varying ammo quality and pressure.  But for now, I'm ok with saying the cheap/underpowered ammo is for use with the beater carbine length gun.

I suppose the question would come to what if you had to pare down your collection, which would I keep between the midlength and the carbine. . .

You have a 6940, so carbine length.  Any experience with midlengths?

I have the Colt carbine and a PSA upper/lower mid-length build.  I also have a PWS piston and a Noveske mid-length I have yet to shoot.  I have also co-built a Spikes tactical AR with Mid Length which my daughter has.

From a shooting perspective, I see no difference in the carbine versus mid.  If you look at the chart, the two overlap at 14-18 inches.  My 16+" barrels are well inside that range.

Since the PSA is a project gun, I opted to use a mid length for 2 reasons:

1.  I like the extra length for the handguard, for both room to adjust the hold, and
2.  a longer sight radius.

I wanted to try an adjustable gas block to tune the gas system, just for experimentation purposes with ammo types.  Since the handguard is mid length, I figured the longer gas system would put the gas block closer to the front end, making it easier to reach the gas block with the adjusting allen wrench.

These are issues that only have to be considered it you're playing gunsmith/hobbyist and using longer-than carbine length handguards.  For practice and real-world use, either length is fine.

I've shot 223/556 guns with carbine length gas, mid lengths with a vltor A5 buffer, rifle length gas, 300blk with pistol and carbine gas, and 6.8spc with rifle length gas. My favorite by far is 223/556 in rifle length, but it's probably more due to the guns themselves than the gas tube length.

The only one to ever give me trouble was 300blk with carbine length gas being unreliable in feeding and failures to eject.

Pretty much none of them have had much recoil, but the rifle lengths have usually had the least. That might be more due to weight than the gas system though.

I have everything except the pistol length.  My favorite is the intermediate lengths on my 18" SS and 16" SS barrels from Noveske.  I also have a Voodoo rifle length 16.5" that runs well but I don't like the very lightweight or pencil barrel profiles.  I also have several carbine and midlength set ups.  I don't find a huge difference in carbine to midlength in general but I do have some mid set ups with the Vltor A5's that are a noticeable improvement over most of my carbine length set ups.  I will note however that I do have some carbine length set ups that feel better than mids.  It really can be dependent and I do generally built with the Vltor A5.  However there is a noted difference when talking intermediate to a full rifle length.

I have a 14.5 with a mid.  Shoots very well with any ammo I tried to feed it.  I also have an 18" with a rifle length, it is finicky with lower power and cheap ammo.  It does function with PMC, but on that ragged edge.  I use this rifle with hotter loads anyway.  Both seem to have a softer impulse than any carbine length I have shot.  Not an AR expert by any stretch, just my observations on what I have put together.  My 18 has a Viltor A5 flatwire spring, 14.5 has a Maxim collapsible with the captured buffer.