The .577 was much more popular and offered more widely. I think you made the right choice. As for penetration, I have heard stories about the .600 lacking penetration, but it is a trade off between bullet diameter and bullet weight, as they are both pretty much the same velocity. I would have thought that the extra 200 grains of bullet would make up for the extra diameter, but have not done any objective tests! I have 2 cases standing side by side now and the .577 looks no less impressive than the .600 either! .600 rifle would be at least a pound if not 2 or 3 heavier. All in all I'd say the .577 is just more useable, although if it were me I'd have gone with the 2-3/4 case

How about a photo of your .577?