after about 4K rounds of 9mm.  I know 9mm is tough on lowers but I'm a bit surprised.  I have a Lightning Link and link protector in my Colt SP1 lower with a ramped bolt.  I knew something was up when I started to get some failure to fires after pulling of the trigger.  I may have some other problems but I need to fix this one first.  Fortunately, the pin holes still look fine.

My buddy broke his RLL running straight blowback 9mm as well and he also had a KNS protector installed.

4K sounds like a pretty good run for a cheap trigger pin using a Lightning Link type of device.

Well now I'm scared. Next on my list is to setup a PSA AR-V as a RLL host. Any tips you can share? Or things to avoid?

I personally wouldn't run a straight blowback 9mm setup with a RR or RLL.

Appreciate the feedback.  I already ordered the damn thing and its on the way to my FFL now

What would be a suitable alternative?  One of the CMMG radial delayed solutions? The reason I ordered the AR-V is I have a couple scorpions already and prefer those mags over traditional pistol mags in a subgun.  I'm not familiar enough with the CMMG options to know if they can be modified to work with a RLL or not.

The CMMG 'Banshee' originally called the 'Guard' when it first came out uses a bolt carrier similar to a milspec.  The tail end is identical to an M16 carrier so no problem to have it milled back.  However, the Banshee/Guard destroys ejector springs as I believe the ejector spring is absorbing some of the initial recoil impulse.  This also happens on the MEAN Arms bearing delay upper as both systems utilize the same spring loaded ejector found in a typical 556 bolt.  This is not an issue in 556 since the bolt is truly locked.
Here is an active thread on the Rimfire section discussing this issue with the CMMG: https://www.ar15.com/forums/AR-15/Has-the-CMMG-RDB-ejector-spring-issue-been-resolved-by-the-Mk10-spring-/15-781020/
I think someone posted his dying at 300 rounds.  In my experience they usually die around 1K rounds.
Here are links on my website discussing my setup: https://c3junkie.com/?page_id=280
Also my website discussing the ejector spring failures: https://c3junkie.com/?page_id=221

You can see that I am using a custom fixed ejector on my first link but even that has it's own issues as the bolt eventually cracked at around 25K rounds.

All that said, I am working on 2 different setups right now to finally resolve these issues for people that want a smooth 9mm SMG setup for the M16 (RR, RDIAS and RLL).
The first setup ran perfectly.  Ran it at the last SMG match last weekend and a couple friends used it for a few bays.  Approximately 1K rounds through it with only 1 possible malfunction, I say possible because the guy was using his reloads and the round didn't go off.  He ejected the round and didn't recover it so it is unknown if he had a missing / sideways or upside down primer.

I was planning on sharing the details once the 2nd configuration is done but I'm waiting on parts to start the 2nd configuration.  I am hoping to share the details in less than 30 days assuming I get the parts and also have time to do some testing of at least 1K rounds.  Again taking to SMG match, first Sunday of the month.

The indoor ranges that I use only allow subgun calibers.   I intend to continue to use the LL, ramped bolt, link protector and stainless steel pins.      What I will do different though is inspect the hammer pin after every range visit and replace it every 1K rounds or so.  Otherwise the LL is just going to sit in the safe unused.

Really looking forward to your findings and details as far as a true 9mm full auto host (RLL specifically). @amphibian

In the interim, I reckon I might just mill the bolt of this new AR-V and at least give it a whirl -- particularly with all the challenges you indicated with the delayed blowback options.

I will also keep an eye the pins as well after every session once I get it running.  I will also get some stainless trigger pins stat. @mike703

Y'all are awesome. Thanks for all the info.

Sure thing.  This is really exciting for me.  For 20 years, I have wanted a setup that has the smoothness of the MP5 but with all the creature comforts of the M16 and now I finally have it.    On top of that, the MP5 never had a great trigger and is a tad fast in full auto.  I clocked version 1 of my new setup at 604 RPM and it is extremely tuneable unlike the MP5 where all you can do is change locking pieces.

All that said, I think it is a very small amount of people that would be interested in this.

Count me interested :)

Building a 9mm SD upper using the RDB system for my RR lower.

I have the cmmg guard 45 acp, and it has been perfect for me for several years, I have well over 1k rounds on it, zero issues so far.

Yes, the CMMG RDB is fine in 45ACP.  It is in 9mm /40SW and 10mm that have the ejector spring failures....all of which operate at higher pressures than 45ACP.

Have you thought about using an original Colt 9mm Carbine lower? Not the SP-1 based lower. The model just before and the LEO marked lowers until around 1999. Those lowers had a pinned in steel block and used larger than standard fire control pins. I had one with the steel carrier block removed and clearanced for a Lightning Link. I thought that the extra thickness of the fire control pins and the KNS stainless steel pins were cheap insurance. I have since moved on to a RDB with the Mean Arms 9mm incerts for P-mags. Good luck with the project.

Scott