CB's (CB long and CB short are idenctical powder charge and round, just different size case), and Aguila colibri rounds are primer only, with maybe a little powder in there. It's all glued to the bottom. Open a shell up and you'll see it's mostly empty.
They're quiet because there's no powder to make a "bang." Or at least barely any.
Subsonics are just a lighter (slower burning) powder, or just not filled as much, so that you don't get quite the bang because you don't get the sonic boom, but they're still much louder than the CB's.
From slowest to fastest in FPS, there's the Aguila Colibri, Aguila Super Colibri, and CCI CB (long or short). But CB's are still about as loud as, as someone put it some time ago, a bug's fart.
Hope this helps. I personally wish they made CB's in a HP round, because as it stands they're quieter than most BB guns, so you can get the critters in your back yard. Just don't plan in killing anything without either 3+ rounds or a headshot (hard to do unless you're sighted in with this round, which is a bear when you go ahead and sight BACK in for your regular rounds). Not even squirrels, I've watched chipmunk after munk scurry off after 1 or 2 rounds in em.
Happy huntin, hope this helps
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