I just bought a silencerco hybrid and I want to sbr my ar next. I want to build a short quiet upper. I am torn between 9mm and 300 blackout. So far here are my pros n cons.

300blk
$200 8.5" upper + adj. gas block = good to go
Subsonic ammo / reloading components are about twice as expensive as 147gr 9mm

9mm
Cheap ammo / components
I have to buy a lot more. Upper, bcg, mag block, buffer, mags, etc

I like the idea of the 9mm, I will shoot more. <$0.20 vs >$0.40 per bullet is hard to ignore. I'm mostly just going to be plinking in my back yard at 50-100yds. I won't be hunting or using it for anything important other than possible home defense if I feel confident in it. In the long run the 9mm will be cheaper, but that will take years with the ammount I shoot.

I will probably do both eventually. What should I go with?

What runs better, shoots better, sounds better suppressed, sets up easier, is better for reloading, etc. i have no experience with either, and all I reload for currently is 5.56. I have 9mm brass and 5.56 ready to convert.

9mm is not a good 50-100 round unless you are shooting IPSC sized targets. I'd say go with 300blk. If 9mm coats you $400 more to build the upper then you get 1,000 more rounds by not spending that. That's a lot of shooting before you buy your first round of cheap 9mm.

I have both. The 9mm is more fun to me.

I reload 5.56 and might start on .300. I would not bother with the price of 9mm loaded ammo these days.

How do they both function and shoot?  

What kind of setup do you have; budget like kak value line or palmetto, mid rsnge, or high end noveske?  Build it yourself or store bought upper?

I have both as well.

Two AR-9s, both colt style (I only own one glock). One with a Hahn block and the other an 80% dedicated 9mm lower. Both SBRs. One has a 3.5" barrel from QC10 with 3 lug and a Rugged obsidian. Feeds 147gr FMJ great but choked on feeding Federal HST for reasons I still need to look into. Other has an 8" (IIRC) barrel with a JP comp on it intended for PCC competition. Still feeds FMJ great.

I have two blackouts as well. One is a factory DD 300S (10.3"), the other is homebuilt from Noveske parts and is 8.5" with a Rugged razor on it. The Noveske is my HD gun after the pistol by the bed.

What kind of accuracy is everyone getting from their 300blk builds? Please post what setup you are using as well. I'm considering a 8.5 PSA upper, but I will spend more if needed. I would like to see what people are having good results with. I would like to buila a nice upper, but if people are getting 1.5 moa out of cheaper barrels like psa or kak, I will wait on the noveske or rainier, woa, etc barrel.

I have both plus a 40 S&W and 45ACP DI.

The 9MM an be very quiet (I shoot test rounds in my backyard 4 blocks from the local PD) but can be trickier to setup. Mine uses Glock mags and works great. 40 S&W is pretty hard to set up right. 45ACP DI from Macon Armory is most fun, but more expensive.

300BO is much better at 50 yds and more, but you really need to reload to get the most out of it.

I've never needed an adjustable gas block except for a 16 in KAK 300BO barrel that was way over gassed.

With my handloads I can do about from 1.5" at 50 yards. 10 shot group. At 100 yards it opens up to about 4". Talking 220gr Sierra Matchking subs from a 9" AAC barrel. 3x magnification.  

I would choose the 300BLK, and did. Quiet and fun subsonic, pretty capable supersonic.

My favorite 300BLK SBR had a 10.5" WOA barrel, Fortis hand guard, Engage Armament upper and lower and a Geissele G2S trigger and would shoot as well as I could at 100yds with the optic mounted with most supersonic ammo. Shooting 200gr REAPR expanding subsonic ammo (available through arfcom: https://store.ar15.com/cat.html?cat=10 .) At 50yds, using an IR Defense Patrol thermal scope, I could get ~1" groups at 50yds, with the limit really the thermal target, and about 1.5" groups with 220gr subsonic ammo, don't recall which brands but I think all of the bullets were the 220gr Match Kings.

I sold the barrel earlier this year and installed a 10.5" WOA 223 Wylde barrel, but wouldn't have if I wanted to reload for the 300BLK. If circumstances change and I had the time to reload again, I would set up another 300BLK SBR.

JPK

If ammo availability and cost weren't factors, 300blk.  But this is the real world. I had a 300blk, I sold it and shoot 9mm now for subsonic.  

147 9mm is 1/2 the price of 220 300blk.

I wanted to go 9 too. I ended up going 300 blackout instead. It's just more versatile. With supers it's deadly but now with the new nobler bullet you can get super performance out of subs. You have to load them yourself until someone else starts loading that bullet but it will reliably expand at subsonic speeds.

To me that is a game changer for the 300 blk as that is where this gun excels is being quite.

what subsonic bullet are you speaking of? the noveske bullet?

nevermind found it. nosler hunting ballistic tip 220gr flavor. do they really expand though?

I have both and shoot a metric asston of 9mm, but the 9mm isn't an AR.

I vote 9mm if the goal is a fun, quiet gun. Plenty quiet, and much less hassle to either buy or reload than .300 BLK. I have a Colt SMG pattern Rock River Arms upper receiver group and mag block, and have not had any issues once I got the insertion height right and properly tuned the ejector (about 5 minutes of work with a needle-nosed pliers). Runs great semi and full-auto.

I have only shot a little bit of .300 BLK, and it was also nice and quiet. Possibly less felt recoil because it uses a multi-lug bolt, not straight blowback like the typical 9mm setups. However, the cost per round just doesn't make sense for a fun gun. If I needed to actually kill things quietly within a limited performance envelope (say, 100-150 yards), then it might make sense.