I have been where you are and decided on the .500 A-Square. I also considered the .500 AHR, as well as the .500 Jeffery and the .505 Gibbs. My rifle was built for me by AHR using the CZ 550 Magnum action and I am very happy with it.

IMO, the .500 A-Square is the most practical (note, I did not say traditional or nostalgic) .500 by far. Brass is easy to come by, since it is based on the .378 Wby. (or .416 or .460, etc., Wby.), and a wide variety of bullets is available, since bullet diameter is .510", and, as you say, .510" bullets are far more common and are offered in a greater variety of bullet weights than .505 inchers. Also, brass is available from several sources with proper headstamping (an important factor for anyone contemplating hunting with the cartridge overseas), and as 500grains has said, the A2 is a SAAMI standardized (i.e., non-wildcat) cartridge.

I can get an easy 2,300 fps with 600 grain bullets in the A2--easy, in that it's a low pressure loading. I can push things (and pressures) and get nearly 2,500 fps, but I decided I didn't need that much for hunting purposes.

I am going to experiment with the Barnes 570 grain TSX and 570 grain FN solid bullets as soon as I can get my hands on some. I should be able to push them to 2,400-2,450 fps and still be well under the cartridge's standardized pressure limit of 63,000 plus or minus psi.

From personal experience, I can highly recommend the .500 A2 to anyone wanting a .500 bolt rifle.