I will quote JD, Under .375 Mag-na-port, over .375 brake; .375 even. I have .338 JDJ with 4 slots a real kitten. Just got 16' 45-70 encore barrel back with Magna-brake. Sure hope it helps. Have 7mm-08 encore nothing needed. Had 14" 30-30 Ackley contender without anything. Did not need anything.