I want to build a 22 upper. I know ppl like 4" barrels bc it keeps bulk ammo subsonic.

I want to have a subsonic gun that cycles reliably but I want a 9" barrel because I want to be able to screw the silencer on or off.

So will subsonic ammo cycle the bolt?

Is there anything else I'm missing?

Personally, I like the 4.5 barrel for it's compactness and only shoot 22 CCI SV or Gemtech subs.  When/if I remove my Spectre II, I use a DPMS 5.5" flash hider to keep the muzzle outside the hand guard.  

My TacSol 9" Kestrel works with the same subsonic ammo.

So you're Kestrel cycles just fine with subsonic ammo?

Do u run it supressed? Does it stay subsonic or does it ever go transonic?

Almost 99.9% of my 22lr shooting is suppressed. The Kestrel suppressed performed just as quietly as our 4.5" CMMG SBRs with the CCI SV and Gemtech subs. I don't have a chrono, but have never heard any rounds "crack."

I have a CMMG dedicated 9" .22LR barrel and bolt kit in a standard upper and a freefloat handguard.  I shoot it exclusively suppressed with a Bowers Paradigm and it will do about 1" ten shot groups at 25 yards.  With bulk ammo, most is super sonic.  With subsonic, I've had 1 round break the sound barrier.  Out of I think 7 subsonic ammo types, two won't cycle reliably.  I don't remember which two, I bought 1 or 2 boxes of every type I could find locally and tried them all for accuracy mainly before I settled on one my gun liked a bit better than others.  But really, most were about the same for accuracy, even the bulk pack stuff.  Everything other than those two types works great.

It is one of my funnest guns and is accurate enough for me and shoots much better than my 10/22.

I personally like the overall size with the 9" barrel.

Here's a link to an album:
https://goo.gl/photos/Mo9hjqxH9QWPARm68

Good luck!

Bitter Bastard

I use a 7" barrel under a 9" handguard.  I simply add a 5.5" flash hider when not shooting with my can.

You can also run a 4.5" barrel and fake can when you want to run minus suppressor.  Works great too and is subsonic when you want to run your can.