11-25-2019, 07:00 AM
Yup, it's probably a 300 Fireball which is basically the same as 300 Whisper (also called 300/221). They are both formed from 221 Fireball parent cases & 35mm long. 300 Blackout ammo might fit in that chamber as well but the Blackout brass might be too thick at the neck & cause pressure problems. Alas, I couldn't find anyone who makes carbon fiber wrapped 300 FB Barrels now-days. That chambering might have been phased out in favor of the Blackout. You might contact Proof Research, Christensen Arms, Carbon Six or BSF Barrels & ask if it's one of theirs.