Looks like some sort of 22
Kind of looks like a 22 hornet only rimless
Bigger than 30cal
338 federal?
8 x 57 Mauser
404 Jeffery
6 prc
Do u mean 6.5prc? That’s so 2017 this is newer…
Is it a 300 PRC?
Bingo 
Based on the 375 Ruger cartridge case, the 300 PRC is non-belted and uses a standard .532” magnum bolt face and headspaces off the 30 degree shoulder. This results in better chamber alignment than standard belted magnum cartridges and allows for improved stacking and feeding from fixed magazines.
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I was only reading about it last night lol
Used in something military, full auto, multi barrel?
Yep a military round and a calibre that is still common.
Unless @no1mk3 wakes up and drops a gem of wisdom (but only from memory) there’s no way (I think) anyone will guess this one without cheating. Just so freaking weird.
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You may even have this calibre.
Is it a Sabot round?
Or for under water?