If you were going to build a new 9mm AR, what are the best options? Pros and cons?

Are there any extra options when it comes to folding stocks than are already available for the general AR15?

CMMG Banshee for me is the best Ive found. Reliable, great ergo's and fun.
Ive also used a Foxtrot Mike upper which I also liked but it was finicky with some ammo and I had to match up the right lower.

No different/better option for folders as you need even more buffer weight with a 9mm.

Interesting, why does 9mm require more buffer weight?

Because it’s blowback, as soon as that powder starts burning, the bolt starts moving, so you need weight to keep it from starting too early, which can cause case ruptures. 5 oz. is agreed upon as the minimum weight but 8 to 10 oz is pretty common. You also don’t want a buffer with weights that move like a std. AR buffer, must be solid.

I think there are a few new bufferless designs out.  If I had the $ that is where I'd look.

Otherwise, happy with my pistol using endomags, FM 8.5" side charging upper.

Quarter Circle 10

OP - start here: Lots of 9mm AR blowback tuning info without having to search for it.

In a nutshell:

The following parts individually seem to solve various problems, or appear to cause fewer problems, with 9mm AR platform builds.

- A ramped, proper weight, 9mm bolt from one of the larger reputable manufacturers.
- A barrel with a generous feed cone. [Macon Armory barrel is highly recommended]
- A carbine-length buffer tube with either: a 9mm-specific, carbine length, sliding-weight buffer, paired with a commercial buffer spacer; -OR- a 9mm-specific, extended-length, sliding-weight buffer.[6.5 - 8.5 oz. tends to be the sweet spot for buffer weight]
- A standard strength recoil spring.
- A standard “mil-spec”-type semi-auto trigger set.
- A standard strength hammer spring and trigger spring.
- A properly machined, in-spec. receiver.

Regarding folders:

The Law Tactical folding mechanism

- 9mm bolts are patterned after the 5.56mm bolt carrier, but they need to weigh significantly more to allow for safe operation of the simple blowback system. Additional weight is usually added in one of two ways: an additional weight plug is fastened inside the back of the hollow bolt, or the bolt is split into two parts with a removable solid heavy cylinder at the rear end.
- To use a Law folder, the bolt needs to be the type with an internal weight plug fastened inside the hollow rear of a full-length bolt.
- The inner diameter of the hole in the hollow back end of the bolt needs to be around .615", and the internal weight needs to be removable (they are usually held with a horizontal roll pin). This is because the Law folder uses a bolt extension that plugs into the back of a standard length hollow bolt carrier.
- The Law bolt extension weighs about 2.0 oz., so it will only make up some of the weight lost (usually 3.5 oz.) from removing the internal bolt weight. KAK makes a custom heavy extension plug (3.9 oz.) that is a drop-in replacement for PCC applications. Some people have successfully trimmed and reinstalled the internal bolt weight so that both the shortened weight and the Law extension fit properly. Others have used a heavier buffer to make up the lost weight.

Been more than very happy with New Frontier upper and lower with LRBHO and their BCG. And takes Glock mags. Used Macon Armory "guaranteed to feed" barrel. About sixteen hundred rounds down range....no issues

Banshee is where's it's at nowadays. I personally do not see the same type of potential and value building an AR9, like I do with an AR15.

With that being said, I prefer the B&T offerings. If I didn't already SBR my GHM9, I'd be waiting for the LWRC SMG in 9mm.

Came here to say this.  
Love my Colt pattern 9mm AR, mostly QC10 parts

Colt pattern.  

Metalform mags work.  Sometimes there are great deals to be found.  I have stacks of them.

Mine is a 10.5" chopped by Adco and bolt ramped by Adco.  It is a bullet hose.

Getting parts ready to build a Colt pattern AR. Finding this
information very helpful.

-DDLES lower, company is now QC10
-Gibbz side charger upper
-QC10 5.5” rail
-4.5” Rainier UltraMatch barrel
-SLR linear comp
-Law Tactical folder
-SBA3 brace
-Stern Defense bolt
-Blitzkrieg buffer

A delayed bolt design (CMMG, roller, whatever) for smooth recoil w/ bufferless operation (allowing folder) would be ideal.  Currently I believe you can choose one or the other, but not both.

Sounds like you would like this: https://angstadtarms.com/mdp9/

I know I'm in the minority but I have zero interest.  I want delayed AND use the buffer tube so I can leverage all the aftermarket tubes, buffers and springs.  There is no way I could have gotten my CMMG RDB to shoot as smooth as it does without being able to leverage the Tubb flat springs and the hydraulic buffer I'm using now.

I also want to be able to pop two pins and swap between 9mm and 556 using the same lower like this:

i built one last month...

Using an psa lower with a stern defense m&p/sig mags... with faxon barrel, faxon bolt and 8oz buffer.. flat side Anderson upper. i had about 850 into it with all parts..

So people talk about building them.. might be cheapier to buy one instead of building one...

I had zero issues with it expect the bho stop was sticking on it. It would totally locked up on the last round..I couldn't disengaged to send the bolt home. Since i was at the range i just counted the rounds and made sure i had one left and just drop the mag and insert a new mag.

After the range time I had to take it out the BHO and file it down a little bit to have it slide smoothly in the slot..

The sig mags seem to run better than the m&p but the sig mags are much more worn in than the m&p..

Ran fine with 124gr rn and tcn along with 135gr tcn blue bullets.. pretty accurate with a el cheapo tasco red dot..

ran 300 rounds and sighted it in for 50 yards..

The only reason i did was i wanted a ruger pc but i wasn't going to pay 800 + for one.. looking back i would have just brought one already made at the price point i was into it for...

Don't buy any Gun Tech crap...

I'd build a Colt DOE 9mm subgun clone just for the cool factor

This..............I used a solid heavy buffer........had an out of battery KABOOM...........blew the extractor off the bolt and the magazine out the bottom of the receiver..........scared the crap out of me too!

Mine has been 100% reliable.

Upper was originally a 16" Yankee Hill kit with Hahn adapter.
Swapped to a cheap 8" barrel.
Yankee Hill Ramped 9mm bcg.
Yankee Hill 9mm buffer
Mean arms 9mm pmag

I use the 9mm buffer for the 556 upper too. Works fine sometimes no LRBO.

In a way this is what I settled on as well.  Tried traditional blowback designs with magwell inserts and suppressed the port pop was ridiculous.  

I purchased the the CMMG Banshee Bolt/Barrel and built my upper on that, then installed the CMMG 9mm Follower kits in a couple pmags and voiala! Runs perfectly smooth with a suppressor and is MUCH quieter.  Accuracy is awesome and hasn't missed a beat with rapid binary fire!

I built mine a few years ago with an Armalite lower hahn block with an 16" JSE surplus upper with SS barrel (comes with ramped bolt) and a spikes buffer. It has never givi'n me any real problems. I started with a carbine buffer but the spikes buffer really smoothed out the gun. The upper was a deal then and I feel a deal now $450 complete upper with a2 front sight. It gets a steady diet of russian steel cased as well as Winchester nato 9mm.

Will a 5.56 AR upper pin to a 9mm AR lower?  Would it be able to fire as a single shot.  I have never had a 9mm AR rifle so I have nothing to compare.

If you look at my post above I can use the same lower with no changes for 556 and 9mm with Endomags and the CMMG delayed blowback system.  Can"t really do that with straight blowback.  I know someone is going to point out that you can use the Endomags designed for straight blowback but you still need to go with a different buffer for straight blowback.
No..but you could easily do that if you wanted to.  If you had the CMMG RDB barrel (which is basically just a 9mm barrel with an extension on it) then use an unmodified 5.45x39 bolt and standard carrier, it would then operate as a single shot.