Re: Suppressing a 9 vs 45?
A) "Knock down" - consider a couple of axioms here - for every action there is an equal and opposite reaction (ie things that are shot by handgun rounds are as likely to be knocked down as those who fire the weapon); secondly what your really asking about is energy displacement. While damage can be measured in terms of ED (in practical terms, the margin amongst 9mm & .45 ACP is not worth discussing); it is more commonly contemplated in terms of the performance of the projectile - penetration & expansion. Here understand that for all practical purposes modern bonded hollow points generally perform in an equal capacity through most mediums. That said, .45 ACP 'ball' ammo is a long time proven "nothing fancy performer" that is subsonic off the shelf in 230 grn.
B) Suppression is based in part on aperture size - smaller is inherently more quiet.
C) While it will have more first round pop, a .45 ACP suppressor may be used pretty well on sub calibers. The new SWR Octane .45 is a user serviceable can that would lend itself well to use on a .22LR - .45 ACP.
D) Depending on your goal, review some data on projectiles, suppression of various cans, and sub calibers through them.
Good luck
