The Octane is a suppressor made in both 9mm and .45 bores. The published specs for it show that at 9mm an Octane 9 is 127dB and an Octane 45 suppresses 9mm to 130.4 dB. Both can handle 300 BLK too; the Octane 9 suppresses 300 BLK to 129.5 dB and the Octane 45 suppresses 300 BLK to 133.5 dB.

So, yes, it appears that for any given suppressor design, the closer to bore diameter it is, the quieter it can be, but whether the difference is significant or not is, I guess, up to the hearer. 3 to 4 dB is, in my experience, definitely noticeable and I would call it significant, others may not think it so bad.

I am virtually certain that, the closer the inner diameter of your baffles are to your bullet diameter, the quieter your suppressor will be. (All other factors being equal.)

For a different acoustic reason, the more space inside of the suppressor to contain and accommodate gas expansion, the quieter the suppressor will be.

So, for a given outer diameter and length of suppressor, the closer the “bore” diameter of the suppressor is to the bullet diameter, the quieter it will be. In other words, shooting .22 LR out of a suppressor with .22 caliber baffles will be quieter than shooting .22 LR out of the same suppressor with 9mm (.355 caliber) baffles.

The fun question is how much louder is a suppressor whose bore diameter is oversized compared to the same suppressor with the bore at exactly the right size. Some people say the increase in loudness for the suppressor with an over-sized bore diameter is negligible. Others say it is significant. I have no idea which camp is correct.