I decided to jump into the .308 AR realm and came across a NIB 18" STAG.

Overall, I'm pleased with the features of this rifle and it was well built.

However, it is short cycling ball ammo.  It runs just fine with premium factory rounds such as FED 168gr GMM, Barnes 168gr TTSX and 168gr Hornady A-max rounds - all without issue.  I'm using both the 10 round P-mag that came with the rifle and I've got a dozen or so twenty round
SS Dura max mags.  

I was initially using some old surplus 7.62 ball, then switched to some 150gr American Tactical ( MHE ) ball.  Both types are almost constantly causing short cycling.  Fired case will eject, but the next round is not fed to the chamber.  All spent brass is ejecting to 3 'o clock.

I did find a gas leak on the carrier key, ( removed the key and fill with high temp gasket seal )
and things have improved somewhat after correcting this, but ball ammo still will not consistently cycle fully.  

What are we thinking?  Under-gassed?  Other issue?  I could find no other readily apparent gas leak.  The gas block is tight and I've got plenty air-flow through the gas tube, through the block and out the muzzle.

I would REALLY like for this rifle to reliably cycle with the ball ammo I have.

Might look into a adjustable gas block.

They have a full warranty that stays with the gun from owner to owner.
https://www.stagarms.com/stag-10-marksman-rh-qpq-18-in-308-rifle-bla-sl-na-stag10000142.html

Call them and arrange an RMA.

For what it's worth they may have sized the gas port to function with 308 loads which are higher pressure than milspec 762x51 ball ammo. If you can get some M118 milsoec ammo, I bet it will run just fine.

The downside to setting the gun up to run on cheap ball 7.62 is it will be over gassed on 308 without an adjustable gas block.

Normlizer:

Yes..  this is what I'm starting to think, too.

It would give me the flexibility between the different loads that I want.  I have a couple cases of the two military ball types and I'm not wanting to dump more money into additional ammo for it at this time.

Lol.. hell, this was one of my main reasons for buying this AR in the first place. I had a SOCOM-16 years ago that I eventually sold and I was sitting on all the ammo I had for it still.

I wasn't wanting to go down the RMA road just yet.  

I have done so with a couple other firearms in years past, ( I had to send a NIB SIG 229 back while I was stationed in Germany during the early '90s and a NIB SA35 back to Springfield late last year, ) but I wanted to resolve this issue myself, ( if possible - another reason I bought an AR.. something that I have the means to work on, ) and after having made a couple of range trips with it, making one small repair and trouble shooting the issue as I mentioned in the OP,  it's looking like the adjustable gas block should do the trick, ( fingers crossed! )

I do appreciate the feedback and suggestions thus far.  Thanks!

This.

So.. an update.

I snagged up an adjustable gas block and installed it.  Checked everything over after the install, ran compressed air through the gas tube from the upper and had good flow out the muzzle.

Went with the manufacturers, ( Shield Arms ) recommended initial setting, then started out with some fresh factory 7.62 listed previously.   After progressively firing a shot, opening up the gas block adjustment one click at a time until running out of adjustment, I was still only getting the initial round that was chambered to fire, eject out to around 3-to 3:15-ish, cock the FCG, but would not cycle back enough to pick up another round from the magazine.  

My gut call at this point: the barrels gas port needs to be opened up.  

Thoughts?  Other options/recommendations?  

I figure if I open up the gas port a little,  ( any suggestions on bit size/brands/tips and techniques/etc, ) I can control it with the gas block.

The gas port in my Stag-10 was way undersized. I opened it way up and installed an adjustable gas block. Rifle now works 100% with all ammo in all conditions.

Put an old cleaning rod in the barrel and use a sharp drill bit to open the port up. With an adjustable gas block, so long as the gas port in the barrel is not larger than the gas port in the gas block, you will be fine.

Once you have tuned and tested the gas block to your heart's content, ensure all the screws are torqued properly and then thoroughly steak all the screws with a starter punch. You can also mark all the screws with witness marks to visually assure the screws are not moving. Check the screws every time you clean the rifle

Very good, thank you!

Once I get the time to sit down and work on it, I'll give it a shot.  

An adjustable gas block can only reduce gas to the system, unless you drill the gas port larger.