I recently got one of the NFA trusts and am ordering a pistol caliber suppressor to use on my 9 mm sig 226.

Figured I may as well go ahead and build an SBR as well.  I have 2 stripped PSA lowers on hand that I could use but I was thinking about getting a nicer Mega billet upper/lower combo and form 1 it to SBR.  

Eventually I will probably want a rifle suppressor anyway but as I understand it 5.56 AR's never really get 'quiet' with any reasonably sized suppressor.  

Since I'm already getting a pistol can I thought perhaps it might be more fun (and cheaper to shoot) a 9mm SBR AR.  

After doing some research I'm not particularly interested in using a standard lower with a magblock I'd rather use a dedicated lower.  I do not reload (yet) so I'm not particularly interested in 300 blk simply due to the cost of subsonic ammo.  

TLDR:  9 mm SBR AR to use with existing pistol can or 5.56 SBR and maybe get a can down the road.  

For what it's worth I already have a 14.5" w/permanent FH so I'd probably go 10.5" on 5.56 to have some significant difference.

I've been in the same boat and done a LOT of research. I currently have a 16" 5.56. I'm wanting to build an AR pistol. I ruled out 5.56 first because of the fact it is quite finicky in shorter barrels , very loud and a lot of muzzle flash.
   Then I thought a lot and planned a 9mm build. What I liked about that idea was a really short 4" to 6" barrel. The drawbacks again are somewhat finicky due the blowback system. To fix that you really need to go very heavy with the bolt plus an 8.5 oz buffer. That's a lot of reciprocating weight! So even though recoil and concusion are not bad a 9mm just shakes a lot and tends to be hard on parts.
    Ultimately I (think) I decided to build a .300 Blackout pistol with a 8.5" barrel. The round is designed for short barrels. Full burn by 8 inches. Similar recoil and better energy downrange than 5.56. Ammo cost is a little high still but it is always coming down it seems as the round is gaining a lot of popularity. It is very effective on deer and hogs out to 200+ yards. Concusion is much better than 5.56.
 Eta ..bolts and mags and fcg's are all interchangeable with std 5.56 parts.

Agree with this. Re: 9mm ARs suck and there's another link within that topic.

Disagree with 5.56 being finicky, loud and a lot of muzzle flash....provided you have a silencer. My advice: If you want 9mm, get an MP5. If you want an AR-15, get it in 5.56. Or ARFCOM this shit and get both.

Cue the ' You know he's right' meme. ^^^^

Was in the same boat some time ago and went with an MP5 and then built a 10.5 AR.

Later added a scorpion because the SBR 9's are so damn fun

I own both and love my AR9 with 7.5" barrel and its not obnoxiously loud like my 10.5" 5.56. Ya you can put a can on the 5.56 but then your right back at 16" again and when you get your first SBR its fun to actually be able to use it in a short configuration (coming from someone with a 5.56 suppressor...).

Some people hate on the 9mm Ar15 but you can build one for $800 that shoots perfectly accurate at 50 yards which is all you're really looking for with a 7.5 inch barrel to run around with and they're fun as hell... Not to mention you can go down to a 5" barrel (plenty of people do on here) to get closer to an MP5 length without having your teeth shatter on shorter 5.56 barrels (even my 10.5, but I hear its ridiculous when you get barrels down lower than 10).

I went through the same decision before.. Just buy both.. You'll have a dozen stamps before you know it

What brand mp5 is that GP?  Since I was a kid I've always had a thing for a PDW mp5

How are the triggers on a semi mp5?  My plan was to run an unramped bolt and a geiselle sg3, although I have read that these can eventually be damaged by the heavier recoil impulse of the 9mm.  Can I get a semi mp5 clone with a trigger that I can 'rip' on?  

I'm not entirely opposed to the idea but I do consider that for the price of a goood semi mp5 I could build a nice 300 blk and get the components to start reloading.  Save all the brass when I shoot 5.56 and then be up and running with 300 subs in some time.  

I've got 2 psa lowers sbr'd. So I don't see any need to get a nice billet one, unless of course that's really what you want. As to the 5.56 being finicky in a short barrel, my 11.5 gun has run 100% since the day after it was first built. I've never personally had one that was a problem, that's out of a 10.5, 11.5, and 12.5. I've also had the thought to make one of mine a 9mm, but there are too many other cool options for 9mm. Like the scorpion, mp5, etc. I don't own a suppressor yet, so yea, mine are loud as fuh, but still a blast to shoot.

Go the 5.56 route and buy something else in 9mm and sbr it it later.

It started life as an MKE, although I've replaced over half of it to get the look I wanted. The factory triggers are great, I can sooo easily get 8 pieces of brass in the air at once. Semi-auto.

The MP5 is more expensive, and it's worth every penny. Speaking from experience, you'll just end up with an MP5 eventually anyway. Might as well do it now.

I've only shot 15~20 different ar's in 9mm.
None of them made me want one.

Currently own 3 10.5's they are all i want to shoot most of the time.  Fired 12k rnds threw another LMT 10.5 before selling it.
Built/put together my brothers 10.5 with rainier match barrel it shoots like a dream.

Seen quite a few other 10.5's never an issue with any decent factory or quality parts gun.

In ARFCOM style, both.  I built a Colt-pattern 9mm out of PSA and JSE parts.  5.5 inch barrel with a KAK flash can and Apex tubular handguard.  It has run 100% with the 124gr round nose bulk stuff.  It is a lot of fun and when my Octane comes in, it'll be quiet too.

My other is a 12" Larue stealth barrel on a Mega upper and Noveske NSR11 handguard.  There's a 1/8" gap between the rail and the can when I'm running my Saker.  Without it, it makes a fair amount of noise but the flash is pretty contained by the Trifecta mount.  

I personally don't like anything shorter than 11.5" on my 5.56 ARs, even for a pistol.  The 7.5" inch ones are just really noisy .22s to me.