Building a shorty upper for my SBR. I only want to use endomags since this will get swapped out pretty regular until I SBR another lower.

Can’t decide if I want to build a RDB, blowback, or get one of these BRN/Matador’s. Are the foxtrot mikes with side charger worth a fuck or crap like some of their other stuff? Just need it to work with endo’s.

Bear in mind there are two different types of EndoMags.
One with the built in ejector to be used with straight blowback setups.  I don't like that version since you must have the mag inserted to clear a live round.

The ejectorless EndoMag version relies on the ejector in the upper which would be 3 different uppers I know of:
1.  CMMG RDB
2.  MEAN Arms bearing delay
3.  Angstadt MDP-9 Roller delay

If it's a 5.56 SBR lower, a standard blowback upper is the cheapest option, but will require a buffer swap, too.  Just the buffer - the carbine spring is fine.  This is by far the most "universal" buffer I've found for any 9mm upper: https://kakindustry.com/9mm-carbine-buffer-extended-gold-top

The FM blowback stuff isn't really crap - they recently updated their barrel feed cones so they feed better than before.  It's not as good as the Mat-9, Macon Armory, or Aero/BA EPC feed cone, but it's an improvement.  The only things IMHO that should be discarded immediately are their buffer and recoil spring.  I bought one of their stripped aluminum lowers as a test platform and it runs well.

I have a Matador Mat-9 upper I'll be testing soon. Reports are that they work, AND you don't need to worry about swapping buffers when you swap uppers since it's all one self-contained unit.  Their feed cone (funnel shaped barrel face bevel that leads into the chamber) is excellent and should feed practically anything.

Building an RDB upper is a good choice, too.  As Amphibian mentioned, they use the ejectorless endomags since the ejector is in the bolt.  Again, no buffer swap needed.

...and Mean arms has their new Exomags on pre-order (I have one on order for testing) that's a complete mag without needing to install a kit in a Pmag.

I havefollowed many of the AR9 threads and the testing.
There is a lot of good testing here to read about.

But so far  I still only have a 8" blowback upper.
Barrel is a cheap combat armory version.  I polished the feed cone.
It has been 100%, never a glitch.  
The upper receiver is a gibbz side charger that I got to reduce supressor blowback.
Blowback still sucks.

I have run standard carbine spring and a YHM 9mm buffer.   The also worked for .223 but not 100% on last round lockback.

I got a AR stoner 9mm buffer that is heavier and longer. Supressor blowback still sucks. .223 upper jams with that buffer. Too long.

I have one ejector endomag.
I am very happy with it.
It has performed perfectly for ~1000rds.
I would prefer the version without the ejector due to the safety factor but until I can find a 100% reliable "delayed" upper, I will stick with what I have.

I love AR9's.
In the meantime, I just SBR'd a AP5 to play with.

Yeah I’ve built several 9mm’s before, all blowback, but all with colt style lowers. I’ve had a few of Rudy’s barrels before as well. My 300 lower is already running a 9mm buffer incidentally enough, since adding a KX5 on it, added back pressure allows it to run perfectly with one.

Thanks for the insight! So much has come on the market since my last 9mm AR.

Yeah the ejector in the mag is offputting.

I tried Endomags back in the day & they were junk. Stern AD9 for me now

How far back and junk in what way?  I have been using Endo's in my blowback 9 for about 4 years and they have been 100%.

A couple years ago. I had 1 destroy a PMAG. My biggest problem is the company told me to F off and refused to even look at anything. Forget about admit they had a defective product etc. They are scumbags. I had to do a chargeback on them.

Well it looks like I’m picking up a Colt mag lower pistol build from the EE offer guy locally after all. I’m still going to pick up a few Endo mags to try the upper on my SBR lower but I will likely just go ahead and stamp this lower. I do like colt mags and anything is better than a Glock mag in these things.

my mean mags work fine, but wish there was a mag loader for them.

I went with RDB and that was a major factor.

1)  Too programmed to remove a mag and cycle the action.  
2) Potential wear on the plastic ejector.

No failures at all so far with the Mean/CMMG magazine inserts.

I have no personal experience; but from looking at the photos, it appears that a Blind Squirrel Enterprises mag loader might also work for loading Endomags.

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