Don't know about using 5.56mm in a 9mm can... That may not be the best idea. Most 5.56mm and 7.62mm suppressors use inconel blast baffles to save wear and tear on the rest of the suppressor. They are generally made different in other ways too.
I just got four AAC suppressor mounts for an M4, a Grendel, an FAL, and a .300BLK, all to use their SDN6 suppressor. I understand it will suppress the M4 similar to their older M4-1000 model, and that it will reduce standard 7.62 and Grendel by 30dB's or so. It will suppress the .300BLK with subsonic to a level quieter than the MP5SD --or so they claim.
I've used suppressors on M4's in the army back in '03. They were small and I don't recall the make. They made the M4 sound a lot like an unsuppressed .22lr. Not silent or Hollywood quiet, but they were much better than without. These were ranger models, designed for scout operations where silence isn't as important as masking location. The infantry scouts seldom fire anyway, since that means they got busted in their mission. They used them mostly to break contact and get away while making it harder for the enemy to follow.
For QUIET, you should seriously consider abandoning the M4. Not that you shouldn't suppress it, you have your trust like I do, it is just that you can do a lot more now.
I did a lot of research and ended up going with the AAC SDN6 because I can use it on a lot of different rifles, including the 5.56, and still get that unsuppressed .22lr. signature on the 5.56mm, the 7.62mm (which you can get special subsonics I understand) and the Grendel, which I understand cannot be made subsonic, at least at the moment. The plus is that it can be used to make the .300BLK into a super silent rifle capable of firing subsonic 240gr. bullets accurately to 300m and can fire supersonic ammo rivaling the AK47 (it is 7.63x35mm). Awesome. With the other mounts, flash hiders, muzzle brakes, and a combo flash hider/brake, you can mount them to just about any .30 calibre and under, including the M14, AUG, SIG, FN, you name it. Pretty versatile for one can, and it is a great one at that.
Later on, I may get the dedicated M4-2000 for just the M4. All it is is the SDN6 with smaller holes in the baffles for the 5.56 and under only. I hear the only difference is that this one is not as deep in sound and it is like 3dB lounder --which is supposedly undetectable by the human ear.
As for subsonic ammo in the 5.56mm, I think that is a bad idea. The round just doesn't perform well at low velocity. The round is designed to tumble and break apart (fragment) in soft tissue. It makes nasty wounds at high velocity. At low velocity, it tends to just punch a hole not unlike an ice pick. Coupled with low mass, and you literally have .22 air rifles that are quieter and have near equal power.
Use your 9mm YHM can on a 9mm. If it is for a 9mm rifle, use it on any multitude of hosts. A decent 9mm AR would be a good start. If you can find the Glock mag lowers, those look promising. Oly used to sell them alone, but now you have to buy their whole 9mm rifle, they don't do the best machining or quality control (and I've heard of a few KB's using their uppers due to firing out of battery --they make them real close here). Someone else does make them though, just the lower, one of the Glock makers I think, check Glockmeister. One of those and a lower kit, along with a good 9mm upper (Colt or RRA maybe, there are other good ones too, I'm just not familiar with them). Or you could get a decent 9mm upper and a good mag block that allows you to use VERY cheap Sten mags... I saw a crate of 100 for $50 recently!!! New in cosmoline!!!
You can also use that YHM on a 9mm bolt action, making a VERY quiet 9mm rifle. S&H Arms also make an integral suppressed 9mm Marlin Camp rifle, semi auto.
If it is for a pistol though, well, you'd be better off going that route.
Bottom line is that you should call YHM and ask them for better advice than what you can get on here. But I'd surmise that they'd tell you to keep it on a 9mm or lower. Suppressors are designed to handled the pressures or lower than what they are lablled for, seldom will they do better than that.
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