I've read 9mm uppers have been notorious in damaging lowers in full auto. Something about the impulse slowly wearing out the pin holes.

Is this still the case or has there been solutions made for this? I am waiting on getting my first m16 and would love a 9mm upper. This m16 isn't just a beater though, so I wouldn't want to put anything on it that would eventually damage it.

Thanks

A ramped bolt, KNS pins and a G.I. FCG are your friends.
The buffer should be on the heavy side.. And have sliding weights.

I hever had problems with my registered Sendra.

I used to run a full auto straight blowback 9mm which did egg out my hammer pin holes in a RR SP1 that I used to have.  I only used ramped bolts and heavy buffers still happened to me.
I now run a CMMG 'Radially delayed blowback' setup which is very soft shooting but it does have its drawbacks which I've extensively documented on my website: https://c3junkie.com/?page_id=164

However, if you haven't bought anything yet, I'd wait to see if MEAN Arms delivers on their new 'Bearing Delayed Blowback' system.  
In this video, they are saying it will be released this quarter....(end of 2021)....




I've been in communication with them and confirmed that it is fully compatible with a RR or RDIAS....bear in mind that the new JP roller delayed blowback setup is NOT full auto compatible.  The JP is also only available as a complete lower as the ejector is like a real HK that pivots and the back end is where the auto sear is which is why it won't with a RR or RDIAS or RLL.

Thanks for the info guys.

Something that I was thinking about.... KNS pins are advertised specifically as being strong. Which sounds like the opposite of what would cure the issue. Couldn't this issue be cured by having WEAKER pins? (Not more brittle, but rather more mailable). That way any damage that occurs is occuring to the pins which can be cheap and replaceable. (The same way plastic will break before metal dents, no matter how hard you hit the metal with it).

That is actually what Colt did to their 9mm specific lowers. They use stainless steel fire control pins. The SS pins tend to bend before they break. The standard steel pins tend to snap as they are more brittle. That is the problem. If the hammer pin breaks,  the gun might still function. The notch in the center of the pin will center the hammer pin by a wire detent. But the  pin being thinner in the center, that is where the pin will tend to break. With the break, the pin halves aren't square to the receiver and wallows out the hammer pins holes in the receiver. The two notches in the fire control pins is to retain the pins. The KNS pins don't have the notches. They are held in place from the outside.

The RDB has been well documented by "amphibian". His site has a lot of information. It still seems that CMMG has not gotten the ejector problem fully addressed. A lighter spring and a hydraulic buffer can really help the standard Colt 9mm blowback system, along with a ramped bolt including a tungsten weight. It will never be as smooth as the roller delayed blowback system used in the MP5.  The greater the mass moving in the firearm, the more the firearm will tend to move.  YMMV.

Scott

The KNS pin 'hammer' pins still have the groove in the middle and I wish they didn't make them like that since as you mentioned it is a failure point and if they are holding the pins from the outside, they can't walk out anyways.
Look closely at their picture below and you can see the groove is still in the hammer pin.



They don't have the groove for their 'trigger' pins.


I used to shoot a lot of full auto 9mm blowback and 7.62x39 in my M16 variants and RDIAS hosts.  
One time, I had the hammer pin break in the middle unknowingly, when I went to chamber a live round off a 7.62x39 mag, I had an AD.  Since the hammer pin was broken, it was able to go up, out of engagement of the trigger so it let go of the hammer.  Thank God I had the muzzle pointed in a safe direction.

These days, I run an MGI lower.  With the MGI lower, the magwell covers the head of the hammer pin so I can't use the KNS non-rotating pins....however I now only use 'trigger' pins for both the hammer and trigger.  Since there is no groove in the middle, I have never broken a pin for over a decade.  

For a RR, I like to use the quick change non-rotating set that I no longer see on the KNS website and I flip the orientation so that the 'trigger' pin is used as the hammer pin since there is no groove in the middle.  It can walk out though but I keep an eye on that RR Colt lower.

I even asked KNS about this once.  Why do you put a failure point (i.e. groove) on the non-rotating and captured hammer pin.  The response was something along the lines of our groove is much reduced compared to a standard pin (it is) and they claimed it was still necessary to keep the hammer aligned to the center of the pin vs. using the inside wall of the receiver to keep it aligned.  Not sure I buy that as its not like there is much in term of extra space between the interior walls of the receiver and the ends of the hammer pin holes.

I always considered making a stainless replacement pin for my KNS sets that didn't have the groove, but to KNS credit I have never broken a KNS pin in either 7.62x39 or 9MM.

One other issue to consider running a 9mm blowback setup on a M16 Registered Receiver is you can damage the LRBHO pocket as well given the bolt (and buffer) are much heavier and also get a sizeable acceleration run at the LRBHO before it catches the bolt/carrier as the 9mm combo bolt/carrier is shorter in overall length.

My main 9MM dedicated lower as a dented LRBHO pocket from years of a blowback bolt hitting the LRBHO.

This is mitigated a couple different ways.  You can add a spacer in the buffer tube so that the bolt doesnt get than running start at the LRBHO lever, the downside being additional ROF increase as you are effectively short stroking the bolt carrier with the spacer.

Alternatively you can run mags with the LRBHO tab on the follower deleted (or use modded Uzi mags) so that it never engages.  If I ran a 9MM setup on a M16 RR, this is what I would do.

Maybe I just have bad luck but I have broken at least 4 or 5 KNS SS hammer pins.  

Never broken a KNS trigger pin used as a hammer pin with my MGI lowers and RDIAS.

For 9mm in my RR M16, I eschew the LRBHO.  I use either a long discontinued VM-Hytech adapter for unmodified Uzi mags (been doing so since 2008) or more recently use a Glock 9mm mag adapter that lacks a LRBHO trip (IIRC, a couple of such adapters are sold).  Also, I do not use a buffer tube spacer, and have yet to encounter the unfired round or empty case caught in the trigger pocket that advocates of the spacer bring up.  Which begs the question, if you're getting proper feeding, extraction and ejection, how do you get an unfired round or empty casing into the trigger pocket?

MHO, YMMV, etc.

"One of these days" I'll put together the gathered components for a 9MM build for a RR lower.
From past research ( IIRC a lot from amphibian and some directly from Blitzkreig ) the chosen components are a Kynshot RB5007 hydraulic buffer, Springco red buffer spring, Colt 9mm bolt, drop in mag adapter ( dont want to mar the RR with Allen keys that some adapters use ) DOE upper, and KNS pins.
Nice to know to reverse the pins from this thread.
Modify the hold open catch on the Colt / clone mags if necessary.
My plan is to assemble an upper and test before putting it on the $$$$$ lower.
Dedicated 9MM hammer ?

Brownells mags are supposed to be top notch. I'd have to cut the hold open tab.
https://www.brownells.com/magazines/rifle-magazines/magazines/ar-15-32rd-colt-style-magazine-9mm-prod57092.aspx

IMHO....no straight blowback 9mm M16 will ever be as smooth as an MP5.  Everytime I compared whatever configuration I had to an MP5, I felt disgusted and just wanted to throw those straight blowback parts in the trash.

As mentioned on my site, the tuned CMMG RDB is very good and I think has finally rivaled the MP5 for SMG matches.
LRBHO, completely ambi if desired, easy to work on, way better trigger and tunable cyclic rate.  I have mine in low 600RPM.  
Running the 40SW RDB bolt for 9mm suppressed.  This increases the delay and drops the cyclic rate some.

I shoot for fun not competing in matches and actually prefer the pucka pucka back and forth of an UZI over an MP5 for example.
In my eye a DOE 9MM Colt is super sexy !!!

I too am a sucker for the 9mm DOE's good looks.  I have a replica handguard and upper.  Just need the front sight HW but would like to build it out as a delayed blowback.

Nobody has tried a Gibbz or other side charging upper on a RR? I’m planning to with a reduced power spring and 11.5 Oz worth of buffer and a Stern Glock adapter. I’m thinking of swapping the bolt weight for tungsten. My buffer has a Blitzkreig weight in front of it reducing the bolt travel so I won’t be getting LRBHO. This combo has been great with a binary trigger, how do you guys think it would work with FA?

To each his own but that is a lot of reciprocating mass.  
I do not like a bouncy SMG.  I also wouldn't do that on a transferrable full auto lower.  (I did it a long time ago on a RR Colt SP1 and egged out my hammer pin holes).

The configuration I generally use:
Colt M16A; Colt 16 in. 9mm ‘pencil’ barrel installed in Gibbz Arms G9 (side charge U.S. patent no. 9310146 pattern) upper receiver (ground for magwell adapter clearance on front, inner, longitudinal edge of lips); standard, mil spec fire control group; Spinta Precision 9mm, ramped,  direct blowback bolt carrier group (15.6 oz.); standard, mil spec, six position carbine length buffer tube (i.e., lower receiver extension) without spacer; standard M4 pattern stock with rubber butt pad; standard carbine length, round (piano wire) stainless steel recoil spring and B & T AR9 AR15 hydraulic buffer; Sylvan Arms (i.e., no LRBHO trip) Glock 9mm magazine magwell adapter; ETS 31 round magazine; or VM HyTECH Uzi magazine magwell adapter; 25 round Uzi magazine.

ETA: FWIW, Rate of fire with American Eagle or Blazer Brass 115 gr, FMJ ammo hovers around 700 rounds per minute.
Another ETA: I do have the KNS pins set installed.  Also, for shooting 5.56x45 (through a 16 in. bbl. using a standard mil spec BCG), the recoil system used is a SpringCo Red recoil spring and a KynSHOT  RB5000HP hydraulic buffer which yields an ROF of around 770 RPM.

Disclaimer:  I have no affiliation with or financial interest in any vendor or manufacturer; and I am not a competitive shooter or a blogger who receives any form of compensation for endorsements or favorable public or private comments.  Use of vendor and/or product brand names, if any, is for informational purposes only.

Best of luck.
MHO, YMMV, etc.  Be well.

I guess I’ll just do some limited testing with the blowback and wait for Mean Arms to release their roller system.
In the meantime, what about Scheel’s roller delayed buffer? Anyone have experience with that?

I did this a bit over 2 years ago. DOE clone using CMMG radial delayed setup, it's my favorite upper for now.