I'm surprised nobody mentioned the CMMG Guard radially delayed blowback setup.
I'm running that in full auto with RDIAS.
Also have a full auto MP5 which I think is the gold standard for a closed bolt SMG in terms of smoothness.
I also have a Sig MPX with a post sample DIAS.
I still give the MP5 a slight edge over all of them in terms of smoothness. But I'm not done tuning the CMMG Guard.
My current state is that while the MP5 is slightly smoother than the CMMG Guard, the MP5 is limited in regards to what you can do with it. With the advancement in buffers, springs for the AR platform constantly evolving as well as the myriad of aftermarket parts I much prefer the Guard setup.
I'm using an A5 buffer, 9mm Blitzkrieg hydraulic buffer and 556 Tubb flat wire spring at the moment and it is very smooth. You can see I have full auto MP5K PDW in the background for comparison. http://c3junkie.com/?page_id=538
BTW, I have a buddy running the Guard setup on a PSA Glock lower, all he did was remove the ejector from the PSA lower and made no other mods to the lower.
Bought just the barrel and BCG combo direct from CMMG, using same buffer/tube/spring setup as me and using a RDIAS as well....so another option for you if want to keep using the PSA lower w/ Glock mags but want to get away from straight blowback, but still have the great AR ergonomics.