For 500-600yds, there's no real need to change what you have right now. If its shooting tight, and you are happy with the results, stick with it.
Pretty much anything from the 155s to the 185s should work no problem - just at that distance, there isn't really any overwhelming benefit one way or another beyond what shoots best from your gun. The heavier stuff (200+ gn) starts getting into somewhat more specialized loading technique, and more recoil for minimal benefit @ mid-range.
Personally I like the B155.5BT... shoots very well and is pretty easy to work up a good load with.
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