9mm M16 SMG Options – RR / RDIAS / RLL
These are my opinions and experiences with the options ‘available’ today.  I mean no disrespect to any manufacturers mentioned here and am grateful for having choices available to achieve my goals. Nothing is perfect but the beauty of the M16 platform is to use all the various components made by all the different manufacturers
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If you are reading this I presume you are after the same thing as me: the smoothness of the MP5 but with a nicer cyclic rate (in the 600’s) and if you own a transferrable, you obviously don’t want anything that could damage it or make swapping back to rifle caliber a chore.

March 2024

Option 1: Straight blowback
Pro’s:
1.  cheap and parts are plentiful
2.  easy to tune w/ various buffers and springs but trying to slow it down means adding more reciprocating mass which makes the gun bouncy in full auto
Cons:
1.  Hard on lower (Maybe you don’t care if you are running a Post Sample or RDIAS / RLL), however I have a friend that ran a RLL with a KNS protector in 9mm and his link broke.  I egged out the hammer pin holes of a transferrable Colt SP1 back in the day.  I wouldn’t run it unless we are talking a RDIAS or a post sample.
2.  Need fixed ejector in the lower which makes caliber swap longer.  You can also use the MEAN Endomags (and upcoming Exomags that have the ejector built into the mag, however now you need to leave the mag in the gun to clear a live round).  I personally do NOT like that change in manual of arms at all.
3.  Does not suppress as well as a delayed blowback system
4.  Does not shoot as smooth as a delayed blowback system
5.  If you do decide to use straight blowback, I highly recommend that you refer to ‘blowback9 / droppoint’s’ website here: https://blowback9.wordpress.com/2022/12/06/ultimate-gentle-recoil-9mm-ar/  Note that you need mass for this and he ends up with 25.7 oz of total reciprocating mass.  In my current preferred option 8 below, I have a total reciprocating mass of 16.3 oz.

Option 2: Straight blowback w/ Maxim RDB buffer
Pro’s:
1.  Use your existing straight blowback barrel and magblock if applicable
2.  Should be softer on your lower or RLL than Option 1 but it is relatively new (late 2023 release) so not many people running them in full auto yet to really know.
Cons:
1.  Harder to charge the bolt since you are at a mechanical disadvantage.
2.  Need fixed ejector in the lower which makes caliber swap longer.  You can also use the MEAN Endomags (and upcoming Exomags that have the ejector built into the mag, however now you need to leave the mag in the gun to clear a live round).  I personally do NOT like that change in manual of arms at all.
3.  It doesn’t suppress as well as a delayed blowback setup or even a tuned straight blowback.
4.  Cyclic rate is relatively high and as mentioned can’t really tune it.
5.   Can no longer add/remove weight to the bolt or use the buffer tube to tune it.
All that said, this may be a good option for a Colt DoE 633 upper that isn’t suppressed in it’s factory form as well as the Freedom Ordnance FM-9 belt fed upper

Option 3: Freedom Ordnance FM-9 Belt fed upper
Wasn’t sure to mention these since they are so unique being belt fed and as of 2024, I think they are about to get re-released soon.  These run great out of the box.  Yes, they are straight blowback but I think that the energy required to pull the belt on the rearward stroke helps to keep the cyclic rate low and smoother than a typical straight blowback 9mm M16. I now use mine with Option 2 above with the FM-9 and it smooths out the FM-9 even more and with the delay induced with the belt fed mechanism combined with the Maxim RDB it suppresses very well.  Note that it suppresses well without the Maxim RDB but used in conjunction makes it a great combination. I have always run mine with a post sample or RDIAS.  I have seen people run RLL’s with these so I presume they are fine and the addition of the Maxim makes it shoot smoother.
This picture shows mine is running at 602 RPM suppressed with Maxim RDB.  https://c3junkie.com/wp-content/uploads/2023/08/FM9-MaximRDB-RPM.jpg

Option 4: CMMG RDB (Radially Delayed Blowback) – Initially called the Guard and later called the Banshee
Patent info here: https://patents.google.com/patent/US20180142972A1/en

You can see that it was filed in 2017.  They initially released it in  .45 ACP.  I was primarily interested in it for 9mm so waited to get a barrel and BCG combo and stripped lower in 2018 in 9mm.
In summary, I haven’t had issues with it in .45ACP.  However, in 9mm, .40SW and 10mm expect to have issues with the ejector spring wearing out at about 1K rounds.

Pros:
1.  The only proprietary parts are the BCG and barrel.  An argument could made that the barrel really isn’t proprietary either as the extension does NOT have corresponding angles cut into them like the patent shows.  Only the bolt has angles cut into them.  More on the barrel here.  The carrier is machined so the bolt has a more limited rotation than a standard carrier.
2.  Since it does use the buffer tube, you can leverage all the various buffers and  springs out there for tuning as well as CMMG’s carrier weight kit
3.  Pretty flexible on feeding options, Glock mags, Colt mags, Endomags, upcoming Exomags, Sig mags and some have used the NFA S-9 lower to work with Scorpion mags.  I also assume you could feed it with the Stern mag block using Beretta mags as well.

Cons:
1.  Ejector spring fails around 1K rounds.  You can make a custom fixed ejector and then slot your bolt and carrier as mentioned here but you need to keep an eye on the bolt as it will eventually crack.

Option 5: MEAN Arms (Bearing Delayed Blowback)
Pros:
1.  Very tune-able and can be actually softer than an MP5. 
2.  Suppresses well.

Cons:
1.  Like the CMMG Guard/Banshee, it relies on the ejector in the bolt.  It actually uses a standard AR15/M16 ejector spring like the CMMG Guard/Banshee initially did but later went to a proprietary stronger spring to extend the life of their ejector spring.  Talking to MEAN, sounds like they are relying on the BCG to impact the buffer tube on the rearward stroke.  I believe to aid in the ejection.  If you have fixed ejector, you wouldn’t need to rely on that.  This is also not conjecture.  I purchased a 7.5″ MEAN upper and had sporadic failures to eject early on.  Discovered the ejector spring broke.  Replaced with a CMMG MK10 ejector spring and that went from 10lbs to 4 lbs in less than 300 rounds with failures to eject again.  Maybe if you are not trying slow it down it is fine but that isn’t what I am after.
2.  Currently only available as a complete upper.  They probably won’t sell you an spare trunion and people have asked about getting spare BCG’s but haven’t seen movement on that.
3.  Manual calls out replacing the ejector spring and pin and extractor spring at 2,500 if shooting full auto and / or suppressed and you can see I didn’t get half that far.  At 5,000 rounds you need to do the above as well as change the bearings, FP spring and bearing ring retainer.
4.  The rails that the carrier rides on interfere with using Colt mags and probably wouldn’t work with MP5 or Scorpion mags either.  I think the only supported feeding methods are with Glock mags, Endomags and the upcoming Exomags.

Option 6: Sig MPX – RDIAS only and maybe RLL
Pros:
1.  Very smooth
Cons:
1.   Shoots very fast – over 1000RPM
2.   Not really tune-able – yes you can get a ILWT gas regulator or a custom one or whatever but it won’t slow it down significantly
3.  IME, didn’t suppress well with a muzzle suppressor, I was told that they do suppress well with an integral suppressor but the high cyclic rate and lack of tuning was a deal breaker for me.
4.  Almost all proprietary and maybe shouldn’t even be mentioned but I get asked about it all the time so figured might as well document here

Option 7: JP-5 – NOT an option
I list this one because people mention it to me but it won’t work in full auto.  I already reached out to JP and confirmed.  The ejector is similar to an HK ‘rocking’ ejector where the back of it sits right where the auto sear would be for a RR / RDIAS or RLL.  It also uses a shorter upper / lower receiver length so it is a complete firearm purchase.

Option 8: CMMG Dissent hybrid that I came up with
This is heavily documented here but will list a quick summary here about pro’s / con’s.
Pros:
1.  Extremely tune-able
2.  Phase II is compatible with many AR/M16 parts
3.  IMHO, surpasses the MP5 in terms of smoothness with a nicer cyclic rate
4.  Requires no extra work to function with multiple mag options, Endomags (ejectorless version), Colt, Glock, Sig P320.  Can be made to work with some modifications with MP5 and Scorpion mags.
Cons:
1.  Requires a custom made CE (carrier extension) and machine work as documented in the link above.

Option 9: Angstadt MDP-9 – NOT an option
You may thinking, why not just make a CE like I did for Option 8 for this upper?  Even if you did do that it isn’t a straight shot from the carrier to the buffer tube.  Also the rear take down pin area is higher and would interfere with a trip to engage an autosear but may work easier with a RLL.   I did try running one in stock form to see if it would be better than Option 8 and one of the rollers broke and there was some other damage.  They were also unwilling to sell their proprietary trunion separately.  So IMHO, not worth it to try to make it auto if already having issues in stock semi form and lack of available parts to customize.

Possible Upcoming Options:

1.  LMT 9mm Piston – no release date
2.  Upcoming LWRC IC-9 – ‘chamber ring delayed blowback’ – introduced at 2024 Shot Show.  Unknown if they will sell only the upper or if it even full auto compatible. I am skeptical on how effective it is in it’s ability to delay the action with the chamber ring and if it would even work with steel cased ammo.  Was told that for reloaders, the sizing die should be able to straighten the brass out.
3.  9mm piston upper from a company I cannot disclose (I have an NDA on file with them) – I am looking forward to see how well it suppresses as I had a bad experience with the MPX with it’s piston
4.  Upcoming 9mm delayed blowback setup also from a company I cannot disclose (I have an NDA on file with them) – All I will say is that it looks promising but I don’t think it will be as tune-able as Option 8 but will hopefully give it a breakdown if I can get my hands on one.