My personal go to for my CMMG Guard (9mm SBR with 8" barrel) is a 160 Bayou coated lead RN over 3.1 grains of WSF. This won't cycle my pistols but works very reliably with just a standard buffer in the SBR and is quite quiet with my Rugged Obsidian.

My standard 9mm load is a 135 coated lead RN over 4.0 WST. This stays subsonic (about 1000 FPS) in my handguns and in warm weather out of the SBR as well but is noticeably louder, though still overall quiet-ish.

Sorry to revitalize this necro-thread, but wanted to re-affirm El Cid’s info.

It’s funny how everyone tests and talks about which can is quietest and which host is best, but virtually no one discusses ammo.

Some factors to consider with ammo are:
• Sub-Sonic (obviously)
• Sub-Transonic
• Gas volume
• Tone
• Cleanliness

The speed of sound changes with air density, thus things like altitude and barometric pressure can effect what velocity the speed of sound is. Generally people try to stay under 1180fps for this.

However, bullets moving through the air at Transonic speed still make a bit of noise. They are beginning to compress the air enough to cause some noise. Thus for “mouse fart” quiet, you have to load slower. Again, air density matters, but 900-950fps is going to be quieter than 1125fps. 800-850fps is ideal.

The less gas is generated by a load, the less work the can must do to dissipate heat and spread out the noise. Single-base powders in the fast to medium burn rate tend to work best. One must still get the host to function 100% though, so I usually use slightly slower powders within that range.

The tone of the can is mostly determined by design, but I feel the load/ammo effects this too. Given the other factors/parameters, I haven’t sorted this one out yet, but I do feel like single-base powders sound better in tone as well. Less of a “crack” and more of a “boom.”

Finally, the cleaner we ca keep the can, the less we have to service it. Cans sometimes work best “slightly” dirty, so a 95-98% clean powder keeps us in that perfect zone longer.

So, what powder works best? I’ve found, in 9mm, VihtaVouri N320 to N350 work outstanding at all of the above factors. My favorite subsonic load is 158gr FMJ-RN with 4.0grs VV N350. It meets all the above criteria, but so does going with N330 or N320 with a lighter 147gr.