I want a 9mm carbine.  I have someone local selling a PSA AR9.  I have also shot a Ruger 9mm Carbine at a falling plate competition, which was fun.

I am more motivated to get a Ruger Carbine to avoid any short barrel or pistol/brace limitations issues.  My other consideration is to build an AR lower that could work in 9mm and 5.56.  It seems that the 9mm Ruger would be close to as cheap as a 9mm upper without the issues of converting back and forth.

Just a quick look shows the CMMG 9mm uppers for $800 and up for the same price I could get a Ruger Carbine.  Any reason to get an AR15 lower and a 9mm upper over a stand-alone 9mm Carbine?

The only good reason for getting one lower with multiple different caliber uppers is, in my opinion, if you live in a ban state or other locale where getting a gun requires jumping through a lot of hoops and licensing. Then just get one "gun" and do the caliber swap when needed.

Based on your criteria, I'd get the Ruger 9mm. If you want to later, get a separate AR in 5.56, but don't wait too long...

You can also get a 22LR upper and practice cheap(er) on the same trigger and stock.  If you build a carbine there will be no issues with brace rulings as you can use a stock.

I already got a 5.56 pistol and a spare upper with a 16" barrel, so I got the basic AR covered.  I figured If I build a second AR lower it could go from my 16" barrel to the 9mm upper.  I live in constitutional carry state so the only delay is in shipping whatever I buy.

We (it is actually in my wife's name) have a MP15-22 so we have .22 covered.  The MP 15-22 was our gateway drug, it led me to building my own AR.  Then a guy at the range let me borrow his Ruger 9mm and actually gave me rounds to shoot it and my own pistol.  It got me thinking a 9mm carbine is also a lot of fun.  Both the .22 and 9 mm lets me shoot at the pistol range which has more fun things to shoot at.


To confirm what it is have been seeing.  An AR15 lower can be swapped out between 5.56 upper and the right 9mm upper, but the buffer springs must be changed between calibers?

Not necessarily.  It depends on a number of factors such as if you want to use last round bolt hold open (LRBHO), how much optimization you want for total firearm weight, recoil and/or muzzle flip, how much money you are willing to spend, etc.
Also, if you go with a single AR lower for 5.56 uppers and 9 mm uppers, you'll need to decide if you want to use a magwell adapter for the 9mm mags or mags with inserts (e.g., Endo mags) - for that decision, five is usually the tipping point where a magwell adapter is cheaper.  Also, Endo mags only come in one size so if you want (or are legally limited) to smaller or larger capacity, the magwell adapter may be a better choice.
I suggest that you use a Pugh analysis for Pareto optimization to make a self-biased decision.
Best of luck.
MHO, YMMV, etc.  Be well.

See my website regarding the CMMG RDB (Radially Delayed Blowback).
Use the same buffer and spring for 556 and 9mm.  CMMG recommends using a standard buffer and spring.  TMK, this is the only 9mm AR option that you can do that with since it is the only one that is commercially available that is not straight blowback.
On my website, I suggest a different spring and buffer that is significantly smoother and also works in both 556 and 9mm.  Then when using Endomags, it is truly an upper swap.
http://www.c3junkie.com/?page_id=280

While it is true that Endomags only fit in a 30 round P-Mag, the insert itself has a limiter that comes sized for 10 round capacity.  You can trim it for 20 or 30, but it is legal for mag limit states.

oops

I got in on the 3 ar lowers, and one 9mm lower and a G19.4 from PSA for like $600

Then I gave them more $$$ to build the things. The AR9 lower was built, but need the RE, and spring buffer and stock, so I picked up a PSA 16" barrel 9mm upper and slapped a matech and ACO on a QRP mount. The PRO's have Larue QR mounts.

We must have shot it 500 rounds a few years ago, with zero issue with my reloads.

Now I'm bored with it, and it take G-Lock mags, and cycles them faster than the G-Lock 19.5

Thanks for adding the clarification. I'm not very familiar with the Endomags. I started into the Glock mag territory with a Glock 19 pistol and have over 20 Glock mags of various capacities and zero P-Mags. So the Endomag made little to no financial sense for my situation and I got a magwell adapter.

MHO, YMMV, etc.  Be well.

No need for the Pugh Analysis.  I just needed more info like you provided.  Namely with the right setup an upper swap and you are good to go between 9mm and 556.  That entices further to go in that direction.

There is mention here of the Endomags but no one is talking about the CMMG mags.  It appears they serve the same function but the Endomag requires the mag to be inserted in order for the rounds to eject.  The eject is built into the magazine.  With the CMMG setup the mag is not needed for the ejection function.

Nevermind,

It looks as though CMMG and the Enodmag are the same product.

https://www.ar15.com/forums/ar-15/Mean-Arms-and-CMMG-9mm-mag-conversion-the-same-/15-739655/

CMMG is rebranded Endomag, but there are two versions.  One with ejector built in for blowback and one with no ejector for use with the CMMG RDB system.