SingleShot,
I can understand your intrest in the .550 Mag since I have one being built right now. The .505 gibbs has history but it requires extensive modifications to a CZ action or an expensive custom action that was out of my price range. I chose the .550 over the .500 A-square or .500 jeffery because, well, it was bigger. Like you, I shoot a lot of mid-range loads at the range and being able to form brass from locally available cases was another advantage. The rifle should be finished next summer and I can hardly wait.