I was actually looking into suppressing an 8 inch MPX, but was unsure which would be quieter. I've been looking online and asking people that have similar setups but no one has a clear answer (which is why I thought to pose the question myself).
Since my goal is to be quiet and not really go to 50 yards and beyond, I guess my question for you is how quiet is your subsonic 9mm compared to your subsonic 300 Black?
Some people tell me 300 Black is comparable to a subsonic 22, other people tell me since 9mm has less powder it's by definition quieter.
Well, it sounds like you have a good bit of money to throw at this project. I don't know anything about the inner workings of the MPX platform. I'm an AR guy. I assemble all of my own ARs and assemble each for a specific purpose with a specific goal.
My 9mm, as mentioned, is a 5" radial delayed system that I run a with a YHM R9. My 300 BO is an 8" that I run with a YHM R45.
The 9mm, when shooting 150gr subsonic with the R9 sounds like a pellet gun, albeit a loud pellet gun. To my ear, behind the gun, the loudest noise I hear is the cycling of the action. One of the ranges I go to does not allow rapid fire. 1 shot per second. When the range officer is about 25 yards away (in that setting) he doesn't notice my rapid firing. People have told me that it is very quiet. When I put the R45 on the 9mm, it is even more quiet.
The sound suppression is about the with the 300BO. Quiet with the R9 and moreso with the R45.
You're not going to get a straight answer to a question like the one you posed because much of it is subjective and there are many combinations that will get the job done.
I will say that chasing db ratings is a waste of time. I've found that what is more important is the tone of the report. Suppressors with deeper tones (although technically louder) are much more pleasant to the ear than suppressors with more high pitched tones (although technically less loud).
On my 11.5" 5.56 rigs, I run YHM Fat Cats. The Fat Cat is a very short can, but has a 2" diameter. It doesn't rate very well and doesn't get a lot of attention, but due to its design, it has a very low tone, as compared to other similar cans. Although it isn't rated as being hearing safe due to db level, it isn't at all uncomfortable in non enclosed spaces without hearing protection.
Equally as important is mounting options. Is the suppressor you're looking at HUB compatible? If not, will the mounting system accommodate your needs? If you're going to tuck your suppressor, you're likely going to want a mounting system without secondary retention.
My 9mm suppressor is tucked into a tubular handguard that I drilled and tapped to accommodate a light and switch. I run a taper mount with no secondary retention. This way, when the need to remove the suppressor arises, I can just use a strap wrench to remove it, if needed.
On the 300BO, the suppressor isn't tucked. I just run a direct thread mount, since I can just put a wrench on it, if I need to.
My perspective on silencers is that they are just another piece of equipment. I know that at some point in time they will wear out, get blown up or otherwise damaged. I stick to bang for the buck. I'm not looking for anything exotic. Just something that gets the job done. The R45 has been a real surprise. It performs very well, within its limitations and is very flexible. I was , at first, apprehensive about going with a can with such a big bore for 300BO, but do not regret it.
