It all depends on how/where you'll be hunting (as most others have said). I'm often hunting deer while I hunt elk, elevations can be 7k ft to 10k+ ft, and mileage usually varies between 6 and 12 miles a day. For unsuppressed rifles I prefer 8lb or less, with a can 9 lbs or less. I can obviously carry more, but I don't see a point and less weight hanging off-center on one shoulder or in one hand while hiking or packing meat is always better than more.
Here are my normal rigs, neither is too bad, but the Kimber is noticeably more fun to carry all day than the X-Bolt.
The latest addition is a bit chunkier than I prefer, but should still be ok.
This past Saturday I covered a little over 9 miles above 10k ft carrying my 9 lb crossbow. It wasn't bad, but I didn't have to pack anything out, and I certainly wouldn't turn down a 6 lb crossbow.
Keep in mind that an 8lb BAR is probably going to be better than 10 lbs by the time you add scope, mount, sling and ammo. Also, I shoot at our local long range quite often, and it seems to me that in practice reliable 500-600 yd shots in wind, from field positions, are a very different proposition than 400 yd and under.