Question for all you AR15 experts. I have a SAOD 6.6 grendal upper and a Rock River CAR lower. Bascially it is a single shot. Bolt won't lock back. I have tried 9mm buffer with same result. Anyone have a solution? Thanks

The 9mm buffer is not recommended for Grendel.
Grendel does like a heavy, conventional buffer however.

My suspicion is the there's something wrong with your gas system.
But, as Dano requested, we need more information about the rifle.
Is this a SAOD-assembled upper ?
What buffer weights besides 9mm have you tried ?
What is your ammo ?

For starters.

I'ved  fired nothing but factory ammo . Alexander arms and Hornady. I'ved installed a troy low profile gas block  no leaks. the car buffer and 9mm buffer gave same results, bolt dosent lock back and the occasional stove pipe. its a factory saod upper and a new rock car lower. I have used two brand name magazines Alexander Arms and Stoner.

Thanks for your reply I am befuddled.

+10

Gas port alignment is the first thing I would check.  Blowing air through the barrel is a great way to check.  Blow some known good barrels so you get a feel for it.  It's a very noticeable difference.  The gas hole in the block is larger than the gas hole in the barrel in most cases, so you have as much as .020-.030 wiggle room.

the carrier and bolt assembly does not lock back mannually either and other uppers lock back perfectly, and its not a mag issue. I'm guessing it these uppers might only work on rifle length tubes and stocks although all pics and articles i've seen and read show car stocks. this is an ar for my wife who's all of 5-4 so i need the car stock for her. this is a walttor lother bbl so head space is not an issue

OK, forget gas system for now and figure out what you have that is out spec in the receiver extension.
Verify correct tube, spring, buffer.

It's also possible that the bolt catch is out of spec or binding.
This is a visual check, make sure that the bolt is traveling back far enough.

If it's not traveling rearward far enough for the bolt catch to engage the bolt, then there is something wrong as mentioned above with the receiver extension assembly.

Some examples of similar issues are....

Buffer tube too short....airsoft product
Spacer stuck in buffer tube.....previously used for a 9mm rig
Rifle spring in a carbine (telestock) buffer tube system.....coil binding.

Is there a difference between mil spec and commercial tubes and could that be the problem because other uppers run fine although there not 6.5s.

Internally mil spec and commercial are the same.

Did you do the visual check to see if the bolt is traveling rearward enough to engage the the bolt catch ?