I've actually decided to go with a 9mm can for my 300x221. Since I will only be shooting subsonic ammo, the pressures generated don't require the solid steel construction of a dedicated 308 suppressor. My HVT weighs 29 oz, which is a lot of weight hanging off the end of the barrel. My 300x221 is light and handy right now, so a lightweight suppressor won't upset that balance. I think the HVT would have better suppression, but it is also longer. That means more time for the bullet to transit the suppressor, and more time for it to strike a baffle...
I went with the Gemtech Trinity 9mm suppressor. It is rated for full auto SMG fire, and it has various back plates to allow a multitude of mounting options. I may find a tri-lock for my 300x221, then I'd have a QD version as well. I don't have the Trinity yet, and I know my paperwork will be delayed. I screwed up and only sent in one copy of the F4....
It will get round-tripped so I file it duplicate like I was supposed to....