An item in The Firearm Blog caught my eye concerning the new CMMG Mk 9 PCC,  which uses a "radial delayed blowback" action.

 Is this the system which uses a rotary movement of the barrel, driven by the bullet being spun by the rifling, to delay the unlocking? As I recall, this was tried over a century ago (possibly in the trials for a US Army pistol which led to the M1911?).

No, it's an AR-15 adaptation to use rotation of the bolt to drive the bolt carrier (rather than the other way around).  It's a fairly unique solution to getting AR-pattern rifles to work more smoothly in pistol calibers (especially .45) without switching to a heavy straight blowback bolt.  But I think it only really makes sense if you're starting from an AR-15.  

Forgotten Weapons did a video where Ian walks through the action, if you're interested: https://www.youtube.com/watch?v=GJTQOhh7KX8&ab_channel=ForgottenWeapons

And CMMG have a description as well: https://www.youtube.com/watch?v=eNiCyfp5nXA&ab_channel=CMMG

OK, thanks. Don't often see anything new in the way of gun mechanisms these days.

Just like the name implies, it was a delayed blowback mechanism which purely rely on friction  and the radial movement of the bolt to delay the opening.

I recall It uses a spring loaded standard AR bolt with the back of the locking lugs cut to a certain degree angle. The barrel extension locking lugs surface was also need to be cut to match the angle on the bolt locking lugs as well so the bolt can unlock.

Surprised you haven't heard about it yet seeing it has been around for quite a while and was mentioned in a lot of articles. it was kind of a breakthrough. well at least something that can be considered new in the firearm industry.

Relying on angled surfaces and friction for "delaying" the bolt, it is about as "new" as the Thompson submachine gun of 1921 (Blish) or the Italian Villar Perosa of 1915. I think there are also other designs. 

Calling such a design "radial", although nothing at all seems to move radially and all real "unlocking" movement is rotational, is in my eyes simply fake news.  

Yes but unlike the original "effect" the thompson was based on, this design Actually works.

Calling it fake news is inappropriate in every respect though. 

Rather, it's one of those situations that shows people just how imprecise and situational much of the terminology in firearms design really is.

autogun said:

OK, thanks. Don't often see anything new in the way of gun mechanisms these days.

Out of curiosity, is there a summary online of the various types (both common and uncommon) and their pros and cons?  

Refleks said:

Out of curiosity, is there a summary online of the various types (both common and uncommon) and their pros and cons?  

Up to a point, it's not difficult to find such info for broad categories of mechanism, but I've not noticed one which goes into great detail. 

In any case, it's a cool solution  as AR-based PCCs were falling apart like it was a competition, who can make a gun that breaks faster. It is truly epic how much wear and tear we see in PCC competition guns folks use for IPSC ,not because of the 9x19 used but because of all the flaws in the engineering of the guns