What's the minimum barrel length to get the 220gr pills up to ~1090fps (with powders usually used, like 1680 or 5744)? I'm building an SBR as soon as my stamp comes through. I'll be using it exclusively subsonic suppressed

You're handloading, so it doesn't matter.  Tailor the load to the barrel.

The bigger issue is having enough barrel dwell to cycle it reliably.  

You are stuck between the sound barrier cracking and reliable function of the bolt when playing the subsonic game.

7.5" are typically the smallest commercial barrels I have seen available.  Some have gone shorter, but you are not likely going to have great performance in both subsonic and supersonic without some know-how.  8" to 10.5" is a safe bet with good barrel makers.

If you want maximum function for subs, open up your gas port and run subs exclusively (you will be able to load lighter bullets subsonic this way that still cycle the action).  It will beat the hell out of your gun if you run supersonic loads through it without an adjustable gas block set to half function though.

there several threads and barrel showing 6.25 blackout pistol gas barrels are running fine   using unsupressed supersonic, supressed supersonic, and supressed subsonic factory loads.

And 6.5" is enough to get the heaviest .30 up to ~ 1100?

Thanks

if your handloading you can load to whatever level you want

Most suppressors are warranted to 8" minimum.

There's only so much capacity in a small case with a long bullet

the 300 blackout can easily be loaded to supersonic and beyond with the 240 SMK, getting 1050 out of a short barrel is pretty easy

Nice!  How short a barrel? Loads?  

Good to know. I need a barrel for my encore and 5" was suggested since I don't have to worry about cycling. I'll probably end up with a shorty AR too since one is never enough