Just wanna say that i have both systems, owned the MP5’s and now own an MPX, Tavor 9mm, Uzi semi, a couple AR9’s CX4, Evo, etc.

I like the Tavor a lot, its got a correct weight bolt for 9mm, and all you really feel when shooting it is the bolt moving back and forth, its not a heavy recoil like most diect blowback 9mm carbines, the MPx is one of the lightest recoiling 9mm’s, however if SHTF id probably grab the Tavor or the UZI just due to the simplicity of the system, how all the parts are large and not a lot of small parts to break.

Also there is something about the UZI that makes it extremely quiet, out of all the guns i have used my 9mm can on the UZI seems to have something in the design that makes it extremely quiet.

Figured that was the advantage. I asked about size because all the guns that take them are pretty fat.

Yeah Im with the OP on this one

Except I think MP5s are perfect and not ancient

Son, we call that a submachine gun.

I agree with op on all points.  You catch hell around here trying to explain to the average troglodyte that a 9mm AR (awful, awful guns) have way too much recoil for what they are shooting. "Hurr Durr 9mm has too much recoil for you!"

I'm in the CZ Scorpion camp, which isn't nearly as nice an MP5, and I think 9mm AR's are terrible as well.

Attached File


I like mine.

Some interesting comments!

Things I would add to the first post...

• If you don't know... You aren't going to learn by reading this thread. Straight Blowback (sucks but) definitely seems fine unless you really know what you're feeling / looking for. Yes, I know you're happy with your whatevergun and that's awesome. I don't think it's justified a 9mm gun has more recoil than a 556 DI gun, but hey, that's the point of this thread. Turn away, because ignorance is bliss in this case. I think I just wanted to see where other people were at with the views I have.


• Full auto and Open Bolt. You're right, I neglected that in my post. Those are acceptable. And not because I'm jealous of your sweet toys. I'll absolutely give it up to open bolt full auto Uzis (heavy and clunky) or Stens etc. I think I have enough time on full auto to say that it's very different from semi in terms of what works best. I appreciated Amphibians post on 9mm RPMs/cyclic rates because it gives me a good idea of the gun's dwell time and recoil (bolt velocity * weight). I think what works in FA can imply SA, but not always tell the whole story. I've shot a full auto UMP in 45 that was stupid, and an Mac in 45 that just purred. I can't think of any scenario that straight blowback is better in except open bolt, because it seems a little dangerous with most delayed systems. I can't think of a delayed open bolt. So, good point someone! There is also the availability issue if you're into full auto, no one should pay $30,000 for an MP5.


• As to why Double Stack mags are better than Single Stack. Yea loading. I think imo the LRBHO is ultimately going to be more reliable if you have a larger opening (this may be why many Glock lowers don't have BHO). I would bet it takes less force (from a bolt) to strip a round from a double where the round is typically touching slick brass and one feed lip vs two lips and the brass under it. More force if it's your thumb trying to strip a round. I'm just going off what is most popular in Sten, Uzi, Mac world where they sometimes have a choice. You could of course make an argument about each round needing to angle into the dual feed ramps but whatever, or double feed malfunctions... IDK. It all seems to work. I think anyone that's used them can agree the ENDO / CMMG mags are kicking ass right now.


• Yes, my post includes a strong indication that suppression is important but... I'm really talking about what is going on at a strictly performance level. The raceguns I've shot recoil heavy but fast. My CMMG seems to recoil slower than my MP5, but both are soft. I can hands down tell you I prefer the CMMG over a blowback racegun because I've timed myself on them. Whatevergun might be super popular with USPSA guys, but you know what else is? Beer bellies and polyester polo shirts If you aren't suppressing, you aren't competing, and you don't care about anything else, that's cool, but the difference is still there. I think that as a group that winds themselves about WHAT-IF scenarios, the softest cleanest shooting gun would be prized.

I guess the over-reaching point is that in this past couple years, a ton of mfgs have released straight blowback carbines and not a single one of them seems to care that their tech is bad. Right now you can buy at least 30 different 9mm AR carbines that functionally are no different than the Colt SMG which was.. unremarkable. These were considered pretty poor running guns when Rock River owned the 9mm carbine market. Now there are at least 5 different Tec-9 clones that B&T is charging top dollar for. And what does the market do?

Eat this stuff up. I'd like to see informed customers demand more. How many SHOT shows do we as a group of consumers need to see come and go with no innovation to be seen and just keep accepting that as status quo?

/Rant off.   I think it's getting there. With the MP5 rebirth, PCC competitions, seriously good options for training analogs out now, it's happening. I'd just like to see it go a little faster. I don't care if you're into your blowback gun, really, good for you. What do I know anyhow!?

I was going to comment on this earlier...actually I love my full auto Open bolt UZI's.  I don't think they are clunky...
If you have them tuned right where the bolt doesn't bottom on your shoulder they are very smooth.  
Most if not all Open bolt SMG's rely on API (advanced primer ignition) so when the primer detonates, the bolt hasn't fully hit the trunion which negates a good bit of the initial recoil impulse.

To contrast, if you take an Open Bolt UZI and even shoot it in semi and take the same exact UZI and swap the bolt for a closed bolt configuration, it will shoot like shit since you have now lost API and the stock just jacks you in the face.

If you get a chance, try a Swedish K, Sterling or Beretta PM12 in open bolt full auto and those are a dream to shoot.  Very smooth.

I didn't bring it up because most people don't have full auto's and open bolt semi's were banned and considered MG's back in 80's....

So we are back to looking at what options are available that are closed bolt....and as already discussed, the options are dismal for anything that isn't straight blowback.

And from what little bit I was exposed too a FA UZI sounds better Suppressed than my Mac why idk.

And shot my Mac and next guy over let off a few burst from a MP5 FA Suppressed and it was night and day. Mine had a distinctive chop and his sounded like a sewing machine.

And I will second the whole 9mm ARs are kinda sucky. I have a dedicated AR9 and it’s ok. It’s recoil was far more than what I expected. Plan on building another 9mm upper using the cmmg guard DBB.

I know people have fiddled with DI .45 AR-based rifles.  Has anyone tried this with a 9mm?  You'd have to seriously skeletonize the bolt "carrier" and probably cut a good bit off your recoil spring.

As i understand it people have tried but there just isn’t enough gas per camber and barrel volume.

Look at why the MPX has its gas port at the chamber.

And also... not to be a fanboy for CMMG, but look at what they did. With only a slightly modified carrier and bolt (and they are slight changes) they got to something nearly as effective as an MP5. What would DI get you over that? Some ability to tune via gas block I guess.

I’m serious, if CMMG markets hard, or licenses the design to others it could replace blowback ARs entirely. I’m skeptical of their ability to make that happen but for $340 for the BCG and barrel it’s already reasonable and a bit of a secret gem right now.

I have a buddy that has an RMW 9mm gas operated upper and it requires reloading of clean burning powder and even then it can't run for a lot of rounds before getting too fouled.

I bought the MPX pretty early on since I thought it addressed all my concerns.
1. Not straight blowback
2. Familiar AR controls and can use OTS grips and triggers**
3. Should be cleaner than the 9mm gas operated with the piston.

Now that I've had an MPX for some time.
1. Yes, recoil impulse is nice but that is about all that I like about it
2.  Yes, familiar AR controls but early on we saw that there are issues using some triggers in the MPX as it has horrendous trigger slap..hence why they put some 'bridge' over the back so the hammer doesn't beat up the back of the disconnector/trigger.
I also still cannot lock the bolt back on the right side like I can with my AR lower.  Another reason I like the CMMG over the MPX as it is literally an upper swap and complete 100% compatibility on parts when it comes to the lower.
3.  Cleaner but sux suppressed...gassy and loud.

I also clocked my MPX (post sample DIAS installed - yes, I'm a C2 manufacturer).  Too fast and I also have the ILWT 4 position gas valve and I think this platform has a small operating windows as it goes from not working to working then to just more rough shooting the higher you go on the gas.

Very interesting regarding the cmmg stuff. I need to look into that.  Do they sound like MP5s with a suppressor?

I've run mine against the MP5. I had a buddy shoot both switching back and forth. They're SLIGHTLY different, but I honestly can't say one is better than the other as a listener. As a shooter, I'm hearing ever so slightly more spring noise in the CMMG because I have a Tubbs flat wire spring and they barely fit in the receiver extension, as where on the MP5 I have the 100º locking piece (proper for fullsize) and I hear ever so slightly more bolt/carrier/tunion (metal on metal) noise.

Toss up. I'd say they tie for gold standard. As where IMO straight blowback guns get the rusty nail award when suppressed (ugh, the POP is clearly not hearing safe).

As to gas, I give it up slightly to the CMMG although both are entirely pleasant to shoot. If I dump rounds as fast as possible with the MP5, I barely sense the buildup of gas, I haven't had even that with the CMMG yet.

Can you expand on this? What do you do to tune them? I would think a heavier recoil spring, or maybe some type of variable weight spring, so the bolt stops its rearward movement before bottoming out.

This is a lot of the reason why the Sterling or PM12S are so smooth. But I haven’t ever shot an Uzi that didn’t feel somewhat choppy. Adding buffers will smooth it out somewhat, but it’s still choppy. Just faster. Nothing like a Sterling.

Wow.  That sounds extremely promising!  And you said they sell that as just a bolt and barrel conversion?  Can they be free with an uzi mag and standard mag block?

What mags work with the CMMG system though?

Discussed several times.  Did a quick Google search and here is one thread: https://www.ar15.com/forums/Armory/Has-anyone-attempted-to-get-a-CMMG-Guard-9mm-to-function-on-an-m16-lower/23-493211/

1.  Originally designed for Glock mags for use with their lower (although some have reported it working with other lowers w/ no mods).  Friend of mine uses a cheap PSA Glock lower with no issues.
2.  Colt mags - I documented this here: http://c3junkie.com/?page_id=597
3.  MP5 mags, some have gotten these to work but bear in mind the width of the MP5 mags which will require an upper clearanced for this....They look cool but no BHO so I think a step backwards
4.  Endomags - in the first link I posted above, I discuss why I like them the best of all the options.

Yes, I initially started w/ an 8" BBL/BCG combo and stripped Guard lower.  A long read, but you can read about all the testing I did on the CMMG Guard for full auto....I have 4 goals for this system and 1 of them is suppression.
I broke the webpage into 4 sections.  Covering 4 different variations in my quest to get a closed bolt SMG to rival the MP5 using the Guard system.
I don't understand what you are asking.

I have a friend that spent a lot of time on this.  He milled weight out of the bolt and also used pretty weak reloads to get his to run very smooth.  I don't know what he did for springs.  I thought it was impressive but I wasn't interested in using special weak loads.

I have run strong springs but then the cyclic rate was just too fast for my tastes.

No I understand you're not trying to troll. Like GI Joe says knowing is half the battle.

I'm sure you've noticed that with a suppressor on your scorpion that there is an increase in bolt velocity compared to shooting it unsuppressed. Because there is nothing but the weight of the bolt and the spring pressure to keep the action of the scorpion closed, an increase in back pressure from a suppressor will increased the bolt velocity of the firearm.

Now with a MP5, because the rollers are being pushed out into the trunnion those rollers are delaying the opening until the pressure has resided and it can cycle. Because it is initially delayed/locked by these rollers, the bolt mass is lighter than relative to a straight blowback bolt. In the application of suppression, the gas pressure is vented out the front of the firearm and there is less gas pressure escaping out the breech.

I tried to explain it to the best of my abilities, but I'm sure somebody else can explain it better than me.

If you ever get the chance to rent or shoot a friends MP5 I highly recommend it.

You've pretty much got it, but delayed blowback is still blowback.  The case begins exiting the chamber upon ignition same as with straight blowback, but the mechanical "lock" created by the rollers and ramp angle on the locking piece significantly slows the speed of the breech/bolt, while accelerating the carrier to the rear at a higher speed.
The steeper the locking piece angle, the higher the ratio of carrier speed to bolt speed before "unlocking", letting you tune the unlock time without adding or removing weight.



The CMMG RDB system is very similar, in that it accelerates the carrier to the rear during unlocking using the cam pin track, but changing the mechanical ratio would required a new bolt with different angles cut on the rear of the lugs.

The Stribog A3 is a little different, in that the bolt mass is significantly larger than the inertia locking mass.  For the sake of simplification, the tiny mass up front is the "carrier" and the larger mass is the bolt.  Tuning the unlock timing here could be done by adjusting the ramp angle on the locking pin notch, similar to the locking piece in an MP5.

I just want to point out the Sten is a double stack single feed magazine, as well as the grease gun.

The original design of the UZI was a closed bolt system, infact the semi auto setup is a slightly modified version of the original FA closed bolt setup, they went to an open bolt to slow the ROF on the UZI down as in closed bolt configuration it ran north of 1000 RPM. Its not like other jerry rigged open to closed bolt configurations.

Attached File

In a thread back in January you had mentioned that Tom Bowers felt the 5" CMMG Guard wasn't as good a choice for suppression as the 8".  I never saw his post since I don't subscribe to that other board.

Obviously blast chamber pressures would be lower for an 8" barrel over a 5".  I'd be more concerned about bolt velocity and port pop.  5" of barrel is plenty to get 147 grn subs up to full speed and makes for a shorter host.

Is there a consensus on issues with or desirability of the 5" vs the 8".

I wouldn't mind the longer host for higher velocity supers, especially if it gains a suppressed reduction for subs at the ear.  Otherwise shorter and lighter is generally better as I'd mostly be shooting heavy subsonics.

Hah...that is my picture there.  That is my .45ACP full auto closed bolt assembly.

You got your info wrong though.  The UZI was originally an Open bolt design.

I used to be a moderator over on UZItalk.com.  Most of the pictures on on the conversion page were taken by me.

The full size UZI came first and it was open bolt.  The Mini UZI was originally open bolt.  It was later that they did both in closed bolt.  The UZI pistol and Micro UZI were always closed bolt only.  (I'm talking factory IMI only.  I know there were open bolt Micro FA conversions but that is not factory correct).

I had all variants in both closed and open bolt.  I sold my Micro a few years ago though.

When the UZI was trying to compete with the MP5 for the Secret Service trials, they went to a closed bolt design which I think is a jerry rigged design from open to closed bolt.  Read page 173 of Dave's UZI book: http://uzitalk.com/reference/uzibook/uzibooksample.pdf

This is 100% false.  H&K teaches the bolt slap in their training courses.

And ya, straight blowback sucks.  It amuses me to see everyone so enthusiastic over their Scorpion Evos, clearly they haven't shot many guns.  The Scorpion is the only SBR I've ever sold.  I couldn't find a single thing to like about it, even ignoring the awful feeling when shooting it.  Just felt cheap.

The Sig MPX is another one that makes me scratch my head.  I've been unimpressed with those I've shot but even if I was Sig seems to continually release half baked products just so the public at large can serve as beta testers.  The MPX is just one of those.  How many different generations and variations of generations do they have now?  Who has the time?

But variety is the spice of life, so I have a UMP (which actually isn't as bad as the Scorpion but still feels like trash next to an MP5) a TP9, and an APC9.  I impulse bought an APC9SD but sold it before I sent in paperwork, I'd rather have an MP5SD.  I have literally zero interest in a pistol caliber AR.

Bottom line, pulling the trigger on an MP5 is a spiritual experience.  You can improve ergonomics, long term durability without maintenance, and come in lower priced.  But when you pull the trigger on an MP5, there is nothing that will touch it.

I don't know if you have been following my posts on the full auto forum and the Guard tuning thread (currently 18 pages long).
Going back to referencing my website, I posted udpates regarding the new .40SW Guard bolt used in 9mm. This link jumps directly to that section: http://c3junkie.com/?page_id=538#section4

Picture below shows two factory CMMG Guard bolts comparing the 9mm to 40SW.  

I have successfully run in the 40SW in 9mm just like how you can use a 9mm bolt bored out to accept 40SW in the UZI and it will still work in 9mm.

Below is what I did myself to modified 5.45 bolts to further delay the unlocking and cutting my own angles before CMMG came out with the 40SW version...documented in Section3 of my site.


As mentioned by a post from CMMG, there is a large operating window on the 9mm Guard.  I think they made the angles more aggressive on the 9mm to handle customers using crappy ammo etc and have the largest operating window.

I think that equates to it running less smooth as it could be.

Again, this is all documented on my site.  I was not happy with how the Guard ran with the CMMG recommended buffer/spring (standard carbine buffer and spring).  I think many will think it is great but compared to an MP5, it is too bouncy.

My recommended buffer tube/buffer and spring greatly helps.  However, I was not happy with how it ran suppressed even the configuration mentioned in Section1 on my site.
Yes, you can get their tuning kit (which is what CMMG recommends, I think 2oz weight when running suppressed)...but that just adds more reciprocating mass.

The better way to go is to again mechanically slow it down via the bolt lug angles.

I hope that CMMG will offer 'suppressor optimized' bolts in the future...just like HK says to use an 80 degree vs 100 degree locking piece for 147Gr suppressed.

However, I think there is a small number of people like me looking for this level of smoothness out of it.  Like my site shows, my first 5.45 bolt I modified and the .40SW bolt will ONLY run suppressed as that is what it is optimized for.  It will NOT run without the suppressor which I'm sure MOST people would bitch about if it doesn't work for both.
I'm not most people.  That is what I want.   This would be analogous to those that want to use an adjustable gas block when going suppressed / unsuppressed.

I'm in the camp of having a setup that is optimized.  I either want adjustable for suppressed / unsuppressed by controlling the gas OR a dedicated suppressed setup with a fixed restricted gas flow where it will only cycle with the can on.

I'm speculating that the 5" would be quieter with a less aggressive bolt angle but again, I don't have a 5" and have zero desire for it as I want more knockdown power and have the same feel as my 556 SBR suppressed setup.

I was with you up until here

(BTW, with 147's, you're only going to gain ~40 fps between a 5" and 8" barrel unless you are tuning with slower powders)

Have you done any SMG matches??
They calibrate the steel poppers HEAVY....you need all the help you can get running 9mm even on full auto..when I say knockdown power I'm talking about steel NOT killing power.

As posted on other threads, I reload my 147gr a little over max charge so Im right at around 1k FPS out of a 10" UZI barrel as that is the longest 9mm barrel I run....I'm not going to load specifically for a 5" barrel.  I want it subsonic for 10" and shorter.

Ahh, that makes more sense

Currrently my only PC AR is a 5" DI .45.
Light (chopped) carrier + DI means it doesn't suppress great, port pop is pretty significant.  No good way to slow it down in its current configuration, as less gas won't cycle, and no room to add mass.
I've considered trying out a guard bolt/barrel, but would have to add some mass back, at least with a long, heavy, solid "gas key" rod to replace the current gas key/charging rod.

It's super fun, and light recoiling, it just doesn't make the best supppressor host.

Attached File

Sorry, Fed with** an uzi mag.  I see above that the answer is yes.

Amphibian gave you the options but just use the 9mm PMAG adapters (CMMG ARC9 / MEAN ARMS ENDO).

Only issue is I stopped a practice shoot cold because a guy was freaking out I was shooting 10y steel with 556.

The advantages of having mags the same size and shape as your 556 are huge. I was able to immediately able to have good reloads because I've practiced so much on 556. I was doing tac reloads between runs while watching people fumble with 30 round Glock stick mags in tiny Glock lower openings. Those mags with the delayed blowback combine make something special.

Amphibian,

Thanks for the quick and thorough response. I hadn't checked out your website in a while.

I hope CMMG is following your work. I'd gladly purchase purpose-tuned bolts for suppressed or reduced rate full auto.

Op is a goober

Straight blow back is best blow back

@Amphibian

How are you setting up and cutting your RDB bolts?
I ghetto-rigged a jewlers vice in the mill and cut a spare .45 bolt to play with and it turned out OK, but not super repeatable.

Neat, i had the wrong inpression then, any idea what makes the closed bolt uzi’s seem quieter than other blowback guns when suppressed?

Inertia of the bolt slamming forwards might delay the rearward acceleration a bit, depending on when the primer is hit in relation to the bolt striking the trunion.
You also don't have the bolt mass stripping a round and slamming home again after every stroke.

Instead of BANG....CLACK  you get CLACKBANG

I did my 5.45 bolt with a Dremel cut off wheel...worked great as it only works suppressed...then when I did my second bolt that works with my fixed ejector, I got impatient and it works suppressed and un-suppressed so not as optimal asl the first one I did.  I'm obviously not doing anything repeatable yet either.  I want to see how these hold up that I've grinded on first.

That hasn't been my experience.  I find my open bolt UZI't to be quieter than the closed bolt as the openbolt is doing API while the closed bolt is just straight blowback.

Haha ok then, I don't feel so bad then

Any obvious uneven wear? I figured if the lugs were uneven they would either fail early or wear in until all of them made contact.

Nope not seeing any signs or wear or even high spots of rubbing....it's just working...

That is what all this talk is about....
I think my CMMG Guard has finally surpassed the MP5.
Extensively documented here: http://c3junkie.com/?page_id=538
As documented there, I don't think it surpasses the MP5 out of the box w/ the CMMG recommended buffer / spring.  However just changing them and we are running very close to the smoothness of the MP5....and as I posted above, when delaying the unlocking further it is right in MP5 territory.
That is the beauty of this system.  A few changes and very tunable.  You cannot say that about the MP5.  You can't tune an MP5 other than change locking pieces which most people don't do.  Can't do much with the springs either.

My video comparing the full auto Guard to my full auto MP5K.
https://www.youtube.com/watch?v=kIMlXOpFop0&t=69s


IMHO, Comparing the Guard to the MP5
1. Softness/smoothness - finally matched now
2. Tunability and availability of parts - already surpassed
3. Ergonomics - already surpassed
4.  Mag options - way surpassed
5.  Optics options- already surpassed
6.  Better full auto cyclic rate than the MP5 as demonstrated in the video link above.
7.  No BHO with the MP5
8.  I can release the bolt and lock it back on the right side on the Guard making for way faster reloads than an MP5.
9.  Dropping mags also faster on the Guard
10.  Enormous trigger options available compared to the mushy MP5 trigger...running Geissele SSF in mine

I think the MP5 days are finally numbered.   Don't get me wrong still a proven, iconic classic...but outdated.

As you can below, all about the same size with stocks deployed.  Only 'disadvantage' is if you must shoot with a stock folded.  I'd gladly give that up to have so many buffer/spring options.

Okay, that was one of the two questions I was going to ask, Now for the first, what other options are there?

So...I’ll try my hardest to wear out my MP5 over the next five years or so, and then buy a Guard-based AR9 when they’re sufficiently refined.

Hope someone takes your suppressed variant of the bolt and makes it a aftermarket part. Don’t think cmmg is too receptive of outside ideas.

I don't know,  my M16 with colt style 32 round mags and Hahn Block functions 100%, and is actually pretty smooth. Not sure I'd take it to battle, but I have zero complaints.

CZ Scorpoin Evo Pistol had been 100% reliable as well and it's pretty fast on plates and I've never once thought it recoiled too much or follow up shots weren't fast enough.

And apparently this has gotten WAY  more technical than I can participate in!

Or get CMMG to sell the 40SW bolt individually.....even you bought a 40SW BCG/BBL combo just to get the bolt it would be cheaper than buying an MP5 clone.

So the 40sw bolt has the correct geometry for 9mm 147gr Suppressed?

Yes....all documented on my site:http://c3junkie.com/?page_id=538#section4

my mke mp5 clone and a friend's pof mp5k shoot a little smoother than my apc9. all semi auto. i think the benefits of the apc9 outweigh the small increase in recoil in my hands. for me the apc9 is more fun to shoot and that is most important to me in a 9mm sub gun.

all i have said is personal preference, of course. im sure some if not most of you get more enjoyment from the mp5 - the z5rs is one of my favorites in the collection. i have a 9mm banshee upper on back order. looking forward to trying that system out.