I really like the FN 5.7x28 upper. Specialty Sports has one in stock for about $750. The way it feeds the rounds is so cool, and you can use a hollow mag for a brass catcher.
I have shot one of these quite a bit and enjoyed it a lot but really doubt its ability at 500+ meters.
.45acp and 5.7 are not going to achieve the posters desire for a 500+meter accurate firearm.