Re: first rifle build
I shoot a very similar rifle:
Stiller Predator action, Jewel trigger, Satern #4 taper fluted barrel, Manners carbon fiber stock.
Mine is chambered in 270 WSM. I am a real fan of the 7 mag, so a 280 improved sounds great, but I have to say this short mag is just the thing for my use. The bare rifle weighs 6.8 lbs, so it is not an ultralight, but is still very portable. With the Nightforce 2.5-10 and Talley rings/bases, it still weighs right around 8 lbs. I am shooting 140 gr. Berger VLD's at almost 3200, and the rifle is accurate enough I have made numerous first shot cold barrel hits on Marmots between 5 and 600 yards this year. Shooting this bullet in this caliber gives decent BC and moderate recoil.
Why do you want to use the light/fast bullets for shooting out to 500 yards? I have been happy with the 140 VLD's, but when the last 200 are gone I'm going to the 150's to reduce wind drift as much as possible. 600 isn't too long range, but it is enough to really begin to see the effects of wind drift and the limitations of lower BC.
What ever caliber you choose, you might want to consider higher BC, lower speed, and less wind drift. Gravity is easy to compensate for, it never changes. Wind is the greatest challenge.