Recently I sent my GUARD 9mm back for failure to eject and bolt over base jams. All matter of extractor and ejector springs were tried along with recoil springs and buffers.

It came back with a new BCG, only visible change was the spring rate on the firing pin spring that holds the bolt open. Others things may have changed but without the old and new side by side I can't see any.

So when a round is loaded the recoil spring pushes the buffer who pushes the carrier, who locks the bolt. The firing pin springs keeps the bolt face against the barrel face.

there is about 0.025 free space for rearward travel before the bevel cut bolt lugs contact the barrel extension lugs. The key cut in the bolt carrier allows that bolt movement without incurring radial transfer of energy to the carrier through the cam action of the bolt key.

So my question is in the milliseconds after ignition, is the case stuck to the sides of chamber or does the case start it rearward movement as soon as it can overcome the inertia of the BCG and buffer?

I think it starts moving the same time as the bullet does.

http://pdfaiw.uspto.gov/.aiw?PageNum=0&docid=20180142972&IDKey=B42D488B654F&HomeUrl=http%3A%2F%2Fappft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO1%2526Sect2%3DHITOFF%2526d%3DPG01%2526p%3D1%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsrchnum.html%2526r%3D1%2526f%3DG%2526l%3D50%2526s1%3D%25252220180142972%252522.PGNR.%2526OS%3DDN%2F20180142972%2526RS%3DDN%2F20180142972

Thanks for posting the patent that description answered all questions.

That initial clearance in the lock bolt is there to provide enough momentum to initiate the radial unlocking. The firing pin spring is there to keep the bolt fully extended during recoil so the bolt is properly rotated for the ejector and extractor to hit the ejection port opening.

It also discusses the angle of locking lugs and it's importance that enough energy be retained to complete the reload cycle also the narrowing of the bolt lugs so the bolt rotation of 22.5 degrees. Also that unlike the DI gun the cam pin drives the carrier rearward.

Update of new BCG. CMMG put a new BCG in my Guard, it runs! However the recoil impulse is noticeable harsher even with 223 flatwire and hyd buffer. Which makes me think they changed the angle of the locking lugs to leave more momentum in the BCG. So now I'll move on to testing a 308 spring and probably a 308 flat wire.

How heavy of a buffer are you using?  Also (just curious) how long a barrel did you get?

I have a 9mm guard with the short barrel (pistol right now, waiting on stamp) and I have a "banshee" in 45acp SBR and both run great.

Saw the 5.7x28mm model yesterday, and the fella tried to tell me it was straight blowback.

The guy at the counter must not be interested in keeping up to date on what he is selling.  Did you bring him up to speed?

Don't really blame him.  It's a special snowflake of a gun out of a few hundred guns he sells.

Snowflake means something different to me I guess.  

Don't get me wrong, I'm really interested in the action, and particularly in the 5.7x28mm.  I just don't expect the average gun counter guy to know the difference btwn straight blowback and radial delayed blowback, particularly when the latter hasn't existed for more than about 5 months.

I don't recall when the CMMG radial delayed blowback firearms first went on sale, but CMMG filed a provisional patent application on Oct. 25, 2016 and a utility patent application on Oct. 24, 2017 which was published on May 24, 2018.  So the CMMG radial delayed blowback system has existed, at least constructively, since Oct. 25, 2016.  Time flies when you're having fun.

ETA:  And an earlier example of a radial delay blowback firearm mechanism that was patented on Aug. 15, 1922 (filed on Dec. 2, 1920) is illustrated and described in U.S. patent no. 1,425,808.

I got interested in PCC uppers at the right time.  Discovered the CMMG RDB system and Mean Endomags before I purchased a blowback 9 setup.  The RDB system and Guard/Banshee upper don't appear to have any problems with the Endomags.   I haven't range tested yet but if they feed reliably I won't be buying a mag block.

Only problems I have encountered so far: The dust covers no longer fit and you can't insert a full 30 rnd mag with the bolt closed.  (Gen 3 Pmags)

HDSledge and I are both running full auto setups.  
Using the 308 spring will yield a higher RoF than the 556 or 300BLK spring.  Not desireable for our goals of having a slower RoF.