I have a Martini Cadet I chambered for the 300 Blackout its a gift for my grandson. That weight of bullet would be subsonic and would not need a gas check buy if you wanted to try some gas checks I could send you a few to try. For all intents the 300 Blackout is a smaller diameter 357 max, in fact I run 357 max shells into the Blackout dies to make rimmed Blackout. Regards Stephen