Reviews of the CMMG MK4 9mm?

[looking4reloadingdeals]

I have been looking for a PCC to use as a trainer for my 556 ARs as my local club does not allow rifle calibers on our bays and our rifle range is static with at least 1 second between shots. Because of this I was looking specifically at the new CMMG MK4s that use the conversion mags. I think that they had a previous version that also used the conversion mags also? 
 

My question however is how reliable are these and why can’t I find a review of them anywhere except for gun youtubers? 
 

IV heard the recoil of the radial delay is close to that of 556 is this true? 
 

Would one of these rifles be a good choice for what I’m trying to accomplish or should I just go with a regular PCC and if so which one?

[GigG]

Do they even make a 9mm 16" gun?  Because come 8/1/2022 pistols with braces are NFA items.

[looking4reloadingdeals]

Yes, they make a 16”, 8” and a 5” 

[Dan Sierpina]

My experience with the RDB system is good and not so good. The good is the recoil is good, you can run a standard AR trigger, and the normal buffer and spring. The not so good, is the ejection issues. To be honest, I've not done any real experimentation with ammo for it, I tend to run hot ammo. Also, CMMG has a tuning weight kit, which I will get to at some point. 

The conversion mags or kits for Pmags run well. Feeding has not been a factor at all.