I see lots of uppers out there in 7.5", 8.5", and 10.5" bbl lengths. Which is best for a 5.56 AR pistol?

Ideally for best ballistic performance... 10.5-11.5" is optimal. Versus what you scarce in concealability and noise going shorter.

Below 10.5 (10.3 for some Colt uppers), muzzle velocity falls off tremendously and blast goes way up. I opted for an 11.5 myself, I think 10.5 would have been fine as well. 11.5 gained me a bit of velocity and should be a bit easier on my silencer.

I haven't personally owned a pistol length gas system but I have read that they are more finicky to get running right with a less forgiving window of proper operation.

I found 10.5 and above to be the most reliable.

Yep. Unless you have some compelling reason to need a shorter barrel (for specific concealment purposes, or the like), 10.5+ is the way to go.

I took a 16” CHF DD midlength barrel and cut it to 12.5” per Dave’s recommendation at Adco. Drilled and pinned on a lopro Geissele gas block. It’s not appreciably longer than my 11.5” SBR but I get no gas to the face and it shoots very reliably. Shooting 77gr IMI SMK, it’s very soft shooting.

OK, the consensus seems to be 10.5". With a carbine length gas system, are there any timing issues with the muzzle being in such close proximity to the gas block? Similar to the original Dissipators' (rifle barrel cut to 16" w/rifle length gas system) timing issues for this reason.

I went with a 7.5, but I only did it because it is a fun range toy. I have plenty of other guns to "Go To" when the time comes. I would go 10.5 or 11.5 for anything serious.

MARSOC is running 10.3" barreled MK18s effectively.
My Buddy commands 2nd Raiders Bn & I been picked his brain prior to my pistol builds.
Based on that I went with 10.5" for 3 of my 4 AR pistols (7.5" is my 4th pistol ) .
No functioning issues at all.

I've had zero issues with the 10.5" PSA upper on my pistol build using a variety of ammo, mostly AE 55 grain, but my last range time I ran through 5 magazines of the Wolf steel case stuff without a hiccup...it is noticeably louder than the longer barrels though, which is a non issue.

As said above, in 5.56 go w/ longer barrel. I do have a 7.5" pistol, and the the muzzle blast is almost as fun as the annoyance. The 3' fireball at dusk is fun, impractical, and louder than it should be.

If you are going to suppress it or use a piston conversion a lot of manufacturers will say anything less than 10.5 or 11.5 will void the warranty ... I tried putting a older Osprey Defense piston system on a 10.5” and it would short stroke, when I called they said the 5.56 needed to be 11.5 or longer ... it worked with my suppressor but my C3 dealer said it would void the warranty ... lesson learned

I went 11.5

quote:

Originally posted by Bigboreshooter:
OK, the consensus seems to be 10.5". With a carbine length gas system, are there any timing issues with the muzzle being in such close proximity to the gas block? Similar to the original Dissipators' (rifle barrel cut to 16" w/rifle length gas system) timing issues for this reason.

It will depend on a few factors:

Size of your gas port.

Ammo.

Weight of your buffer.



I have an H3 in my 11.5 that I shoot with a silencer attached virtually 100% of the time.


It runs with Wolf, (the WPA stuff, not the black box), and all the brass cased ammo I have shot in it.


Who makes your barrel will determine the size of the gas port.

Colt and Daniel Defense generally port theirs to run with proper M193. Other vendors want reliability with whatever commercial ammo may wind up in the gun, so their ports will be a bit larger.

This can be mitigated with an adjustable gas block if you plan to run suppressed.

In 5.56 there is a significant velocity gain going from 10.5" to 11.5" so I went with a 11.5" for my SBR.

That being said it depends on your intended use, all around use I would say 11.5".

I am building a PDW to use as a truck pistol, it will have a 8.5" barrel with a krink FH. depending on ammo ( 55gr TSX or GD) it should be good out to 75m and very handy if I had to use it from the cab.

If you are looking at 9mm or 300blk I would go with 8.5", 6.8spc I would stay around 12".

The velocity loss of 5.56 from a short barrel is why I built a brace of 300Blk pistols for PD use.
Just a better choice for that role. YMMV

Do you have a preference to handguard? Or do you know what rail you want to use? This was what I based my decision off.

I built this 7.5" fun gun and love it. I have a couple of 10.5" sbr's and they are great too.

I had a 6.5" or 7" barrel (I can't remember which now), but prefer 10.5".

Like others, I have a 10.3 and an 11.5. Both have been 100% reliable.