Have you considered the 7.62 Tokarev/7.63 mauser - the mini whisper - factory ammo and heavy bullet load quieter - but power about like 30 carbine I guess - 7.62x39 is more available then the whisper but larger in the base - heavy bullet loads work well. -
Have you considered just working up down loaded cartridge loads for the cases you have? Either lowering the volume (use some epoxy and fill the case up and drill out to the volume you desire) or use an exotic powder like Trail-Boss - (see .308 sub sonic thread here) to down load velocity.
Not that I'm trying to talk you out of doing 300 whisper but it sounds like you would be better off with either common factory ammo or rolling your own sub-sonic in existing cases they you reload for.
Only reason I find for 300 Whisper myself was that I wanted to us AR15 platform with suppressor and subsonic rounds that would work in high capacity magazines without modifiying the gun - my only other choice was something like 458 Socom which was less easy to make the gun for or the ammo
Otherwise heavy bullet 7.62 x 39 would have been my choice as factory ammo for super sonic and heavy bullet loads for subsonic would have been easier but tough to get AR Hi-capacity mags to work with it.
And AK just isnt as accurate as AR.
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