Anything under 11 inches is pretty comfortably under the threshold. Even then it would only risk going transonic in very cold weather conditions. On the ballistics by the inch website, they only test one 147 load, and it does appear to show that type of gradual increase, and then a decrease, but it doesn't get near supersonic speed until about 12-13'. I think that would be the unusual case though. There may be a 'sweet spot' at certain lengths were some 147 loads could go transonic but actually be slower in a longer barrel. It's possible with certain powders that velocity would increase to a certain point and then start going back down with the increased friction/time of a longer barrel. I think its very unlikely that 147's would be supersonic out of a 8' but not a 10.5. I've seen that same disclaimer and wondered about it. like slightly more or less powerful ammo.

I want this thing to operate flawlessly and not just be on that cusp where its operating 'just' well enough that any little variable could make it shit the bed. but Im willing to sacrifice that for reliability. Also, are there any general issues with a 9mm blowback AR cycling that I need to know about prior to diving into this? I want stupid silly quiet. Hopefully this is doable without having to hunt around for 158's (never actually seen them in person. hence trying to as long as possible and HOPEFULLY making it past the end of whatever handguard I choose (or at least flush) lets just say I dont like having my hand infront of a barrel. but then if I decide to shoot unsurpressed. I could always go with a 4.5' barrel and have my suppressor under the freefloat rail. but how long of a barrel can I go with and still keep them sub sonic? Granted. particularly out of pistols (4.5' and less). My understanding is that 148's by in large run at sub sonic speeds. a vast majority of my 9mm stock pile is actually 148 grain stuff (just like the feel of the recoil with those for some reason).

what barrel length should I go with? I know factory 115 & 124's will always be super sonic so I've pretty much written them off. Well gents! I finally did it! Have a double diamond glock lower headed my way and its already going to be pre-engraved and I'll be starting the SBR process as soon as it gets here!

9mm subsonic speed