That doesn't sound too bad. The powder charges for a 300 BO are more than 9mm, but it also has a much bigger case, so I don't know how that would balance out in terms of blast. One big thing in favor of 9mm is the cost to shoot it. Putting an SBR together is probably within 10-15 percent for either one. Feeding them is certainly beyond that.300aac achieves a full powder burn in an 8" barrel, so no concussion like a 223/556 or a 7.62x39 would produce. It might be louder than a 9mm, but not obnoxiously so. Put a blast can or a suppressor on it and it's surprisingly mild.
That was my experience with the shorter 223 as well. Even reloading 300 BO won't help much. Just a 208-grain projectile for subs is more than a whole 9mm round. I typically load 135-147 grain projectiles for 9mm, so they're a little more $$ than 115 range ammo but not much. Unfortunately, based on differences in velocity for 9mm rifle and pistol loads from Lyman, it doesn't look like 9mm benefits a whole lot from longer barrels. I was hoping for a jump like 357 Mag, but no such luck.I hunt with my 8.5" barreled 300 blk pistol & don't use hearing protection so the blast is not that bad. In comparison I have a 7" barreled .223 that I have to use ear plugs & muffs but it's still too loud.