I have a gemtech GM9 suppressor that is rated for 9MM and 300 BLK subsonic. I recently bought a bolt action rifle in 308 win that has a threaded barrel. If the suppressor is rated for subsonic 300 BLK shouldn't it be rated for .308 win subsonic as well since it is the same bullet moving at the same speed? Thanks!

I am aware of the issue of mixing ammo up I'm strictly talking about what the suppressor will handle. If I ever try this it would be monitored very closely.

I have sent a message to Gemtech- no response as of yet.

Thanks for any input you have!

There's more factors than bullet weight and speed. On a longer barreled 308 its probably fine

In general I have found all of the Gemtech GM monocore series to be too short and/or poorly designed to be effective. They are loud and only work well when either shot wet or in the case of the GM-45 used for 300 Blackout. I bought the Trilogy and keep the One on a SBR Blackout where it works and looks good... but almost anything sounds good with subsonic Blackout.
I'm pretty sure that the failure of the GM-45 to work without being shot full of ablative is why GemTech introduced the Lunar baffled suppressor but the Lunar is easily bettered by other suppressors out there.
If you want a monocore suppressor then head over to Liberty. Their suppressors are generally longer but very effective. I have two Mystic X's and a Cosmic. I can shoot my Sig MK25 very fast with the Mystic X as it acts like a match weight or a moderator and the barrel comes back on target very fast.