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Yes, but temperature drops as altitude increases, so there is an indirect effect of altitude.
I read something in a hunting or tramping book years back that stuck in my brain - if I recall...

Yeah, the CCI Quiet ammo is definitely quieter than other subsonic ammo. I have also tried Fiocchi Long Z (I think!), it is quieter than most at 850fps but still louder than the CCI segmented.
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