Josh, curious why the 220 and not the 212? The 220 seems to be designed to support mag feeding in rifles with magazine restrictions and thus has a lower BC than the 212. I wouldn't think you'd be magazine constrained with a custom.The 300 RUM is an elk killer for sure. I'll get it chambered in that round with the intent to shoot the 220 grain Hornady ELD-X. Dallas mentioned he cuts the chamber to optimize accuracy for different types of bullets and has had great success in his past builds.