Yup, proprietary to Bushmaster. There are chamber reamers available: 450 bushmaster chamber reamer Get a barrel of your choosing in .451 from Brownell's or some other supplier. Someone here may be able to help with chambering, maybe other proprietary parts as well. Let's do this!! This unwavering support in helping you spend your money was sponsored by the "Sketch Has Money Burning A Hole In His Pocket" committee, for the small fee of $35.