Looking in to build/buy a 9mm ar pistol is there a good reason to go that way over just buying a cx4 or jrc or something like that?

The 9mm AR shares a lot of parts in common with the 5.56 AR.  Since it's probably not going to be your "go to" home defense/shtf rifle. This means you can use it to try out new/different stuff (sometimes cheap airsoft stuff) before putting it on your main rifle.

Also the controls on  AR are the same 9mm or 5.56. So you don't have to learn something new.

The only real downside to the 9mm AR is that they are not cheap.  In theory a firearm made for a lower pressure cartridge can be made more cheaply and should cost less. Also since most 9mm use the Colt SMG magazine, you will have to buy new magazines.

I sure as hell wouldn't trade either of my 9mms for a CX4.  Matter of fact my favorite rifle is a 5.5" 9mm.

I love my 9mm SBR build on a DDLES lower

glock or colt?

can a jr carbine compete with a 9mm ar?

The CX4 is nice, but whats the biggest mag you can get for it?  With 9mm AR you can get 32 or 33 rounds depending on Colt or Glock mags.  Also, the CX4 has to have a 16" barrel or a tax stamp since it is a rifle, the 9mm AR can be built as a pistol and with a Sig brace still function like a SBR.  I have several SBRs, but I chose this route as I can still cross state lines with this gun without any hassle and it can remain loaded in the truck since it is covered by my CHL because it's a pistol.  That may or may not be an issue for you, but that was a big part of why I went with a 9mm AR.

And you don't have to make the ar USA parts compliant like the storm for a threaded barrel.

I have owned both, a couple of 9mm AR's and six other 9mm carbines/rifles. Been shooting 9mm PCC's for 10 or so years now.
I have a CX-4 (since they first came out), a JRC, a TNW ASR, a Marlin Camp 9, a Hi-Point 995TS, and just for fun a KelTec Sub9 (not the plastic Sub2K).

The CX-4 can not be compared to a 9mm AR. Its totally unfair to the AR. The AR is longer, heavier, cost much more and my CX-4 out shot both my 9mm ARs. The CX-4 is stone reliable and nice and compact. Its got a chrome lined barrel and is supper simple to take down and disassemble. The AR feels like a hog next to that little sexy Italian job. And, the CX-4 is about one half the cost of the 9mm AR. Oh, and why a muzzle device on a 9mm carbine? Serves no purpose other than to drain your check book.

The JRC is more size-wise comparable to the PCC AR.  The JRC uses AR FCG, and furniture.  The operation is not so different. My JRC embarrassed my 9mm ARs at the range shooting rings around both of them.  Cheap magazines, easy to find. A rifle that is 100% reliable and accepts $9 magazines. One thing you give up...a BHO. Seems those don't always work on some 9mm AR's anyway. Oh, and the JRC is far less expensive than the 9mm AR. .

If you want a cool little 9mm look at the TNW ASR. They make it in both a rifle and pistol version. Much lighter than the AR. Costs much less too. Barrel comes off with no tools needed and it fits in your brief case or back pack. AR stocks and pistol grip fit. Glock magazines...Yes, the 9mm AR feels like a fat pig next to this one too.

For several yeas I tried to love 9mm AR's. I put together two of them and worked with magazines and loads. I go them to be reliable  but they would never shoot as well as my JRC or my CX-4. They were too heavy for a PCC, and magazines weren't always cheap. They were never designed to be a 9mm, that came as an after thought and they fiddled with it to make it work. On the other hand, these other firearms were designed from the beginning to be 9mm.

Everyone has an opinion and they are worth just what you pay for them. (as in zero). I can only speak from personal experience with all of these rifles through thousands of rounds over several years use. I find it interesting that some criticize firearms that they have never personally owned. Well, if you are naive enough to believe  them then you deserve what you get. Go out and look around. Pick all the pieces up and see how they feel to you. Try to find someone who owns one and get them to let you shoot it. If you were close by I would be more than happy to bring all of these to the range and let you go at it.

NO, there is not. But there are many good reasons to go with the JRC or TNW ASR or other choices however.

I have a CX4 9mm, a RRA 9mm AR and a JRC .45acp carbine. All the above is right on. I still like my 9mm AR for what it is but both the CX4 and JRC outshoot it. Both Beretta and Glock make 30 round 9mm mags so the Colt pattern 9mm AR doesn't have an advantage in capacity. Best argument for the AR is choices, if you're putting one together there's a ton of different configurations. CX4 is still produced overseas so any mods have to be 922r compliant but i like them right out of the box, all I've done to mine is add a red dot and lighten the trigger. CX4 trigger pull is heavy compared to ARs & JRCs, but in the pistol carbine world you won't find a better trigger than a single stage AR.

I'll chime in here.
Having a 9mm ar it's fun, cost was higher than some of the other pcc's out there.
I still want a cx4 out of all the other choices there.  I already have a beretta 92, and lots of mags though. And they make factory 30 rounders for it, I already have a good supply of just in case they ever stopped making them.
The main reason I want it is for the faster mag changes. Easier to load hand to hand than hand in front of hand.
Cx4's are cheap again now, might as well try one.

I'm surprised nobody had mentioned this but if a DIAS is in your future is highly recommend the AR 9mm

DIAS? Not sure what you mean by that?

Tavor 9mm is pretty fun and super accurate and it takes colt patterned mags:

Reason for a threaded barrel is for my osprey or Evo. I wanted a pcc to shoot suppressed.

That must be one hell of a range day and a whole lot of fun.

Well a friend just got a Sub-2000 that uses G19 mags.  It's pretty cool.  He paid $550 at a gun show.  I will never sell either one of my 9s, but I really want one!

This is all the exact reasons why I would choose AR over a PCC.

Beretta makes a factory 30-round mag.

Still not sure. I am looking at it as a possible project with a ddles lower if I snag one off of GB. That would be a nice starting point. Better than waiting for the mpx that might be vapor ware.

HOT DAMN

If mag capacity is ones main criteria, Beta Co. sells a 100 round double drum 9 mm mag for the Uzi.  One should be able to use that mag in the Uzi variants.  Also, I assume, but have no personal experience, that one could notch the tower to work in AR pattern firearms.  Alternatively, one should be able to use an unmodifed tower in connection with a VM-Hytech magwell block in an AR pattern firearm.  I say "should" because I have a VM-Hytech magwell block, and the one I have is picky about which Uzi mags fit.  Many are too tight, the 25 round IMI mags are usually okay.

Best of luck, and enjoy whatever you decide to pursue.

The trigger makes a big difference for me.   Most 9mm carbines have hard triggers.   With the CX4, the iron sights just dont work for me.   IMHO, the best sights are on the AR15, probably as I have used one since around 1970.  

Also, remember that an AR is like Barbie for men.   You can change calibers, stocks, scopes, etc easily to make a number of versions.

Actually, I really like the CX4 and would buy one.  Only needs a trigger job and a good optic.   I have had the 9mm carbines from Oly, Uzi and Marlin.   I would have the Ruger if they still made them.   But, the AR 15 has them all beat in a number of ways. JMHO

The Shooting Sight after market trigger feels really nice in the 9mm Tavor...crisper, about 4lbs, nice reset.