I have recently been thinking of a light weight long range big game hunting rifle. Fired my imagination to build such a rifle. I think the cartridge would be .300 Win Mag as this can drive the 190-200 gr bullets (with BC of around 0.5) at around 3,000 fps. Rifle will be on a Remington 700 or Winchester M70 action, barrel 26" probably No.3 countor fluted (to keep the weight down), with a HS Precision or McMillan synthetic stock.
Scope will be a Leupold Mark 4 LR/T scope in either 4-14X or 6.5-20X. In light weight tactical type mounts and rings.
I want to try to keep the weight down as much as possible, around 8.5 lbs.
Only accurate rifles are interesting
I have to admit, I am a bit timid of the .300's on a light platform as I don't use brakes (hunt with pointer dogs).
I would opt for either the .280 or .30-06, choice between the two would depend on game weights. If 90% of your target animals are under 180lb, then I would suggest the .280, if heavy animals are the go, then the .30-06. The trajectories would not be as good as the magnums but it would fit the lightweight rifle build and the whole project would come together nicely, filling a niche that you would never regret.
Other guys would opt for a short magnum. Ah, so many choices. We have a smith here in NZ (Dan Hardy) who uses a .300WSM, light contour, 22" barrel suppressed. He won't be getting top velocities from a 22" barrel but that is not the design premise. His rifle fills a niche, lightweight, accurate and most likely produces the same trajectory as the .30-06 I proposed.
I did for a while own a factory Winchester in .300 WIN MAG. Weight was right around 8.5lb. After being tricked up, it was very accurate, shot less than .3".
I used it for a couple of years but re-barreled it 7mm Rem Mag as I couldn't always duplicate the exact hand holds and positions in the field that I needed to ensure the same level of accuracy and POI that I was obtaining at the range. Nevertheless, that was relevent to me only. My shooting buddy at the time was built like a tank, such factors didn't come into play for him.
If you dont want alot of weight, and dont need a magnum, the 6.5x284 is a good choice comes close to a 264 win mag. Depending on distance and game size 257 wby, 270 wby, 7mm stw, lastly 300 win mag, they are all good choices but with the 300wm in a 26" barrel you will be over pressuring your brass and working the hell out of it to get the 210s you should use to 2950 and the 200s to 3000, you really need 2 more inches of barrel.
I think you will be hard pressed to make weight. Tactical rings, and large scope will put you over the top. Get an edge stock, go with some good lightweight rings like the Talley aluminum lightweights and consider a slightly smaller scope.
As for chambering I'd go all out and use the 375 Ruger case necked down to 308. It will hold more powder than the 300 win mag so you can get those bullets to 3000 fps AND its shorter length will allow you to have some leeway in bullet seating depth. I'd also consider adding another inch to the barrel.
I currently run a 300 RUM in a 9 lb package with a 27" #5 profile. When it wears out I will be moving to the 338/375 Ruger and a #3 profile barrel at 26" and will use the upcoming Berger 250 gr VLD. With a projected BC of .762 it would be an even better direction than going with a 30 cal rifle. I'd suggest it now but bullet production is a ways down the road.