I am pretty amazed by the .338/300 Win Mag. I just necked up my .300 Win brass and transfered the 65grs of xmr4350 and loaded a 250gr hornady .338 bullet and the recoil is NOTHING!
I got some superb accuracy with same hornady bullets and 76gr of xmr 4350 (a fairly heavy load, almost fills case), but the recoil is plenty. I figure the .338/300 about duplicates the .333 Dakota and .340 Wby
Now 65 grs of xmr 4350 is just a bit more than a max 30-06 load. I'm thinking that barrel life has to be better with less powder. If a .308/.30-06 can get a 10K round barrel life, with good accuracy for 5k rounds, why not a magnum? After all, its the amount of powder, ain't it, that does the burning?
Anyways, extrapolating from the Barnes manual #3, the .338 has everything in spades over the .30 cals, except maybe the high ballistic coefficients, but the Sierra Matchkings have that covered. Do you need higher BC if you have more velocity and lower pressure?
I know that the only reason the gun/ammo boys don't standardize cases is there is less money in it for them. .338/300RUM is same way. The .338/300Win offers more leeway for seating your bullet way out in the magazine well, which always helps accuracy in my experience.
Anyway, I have a .338RUM and a .30-378; aside from special situations, they are not practical field guns.
I admire the guys who like to tote their crew-served field pieces to the canyons etc, but I have no use for even a 15 lb rifle, much less a 25/30 pounder.
Hell, buy a Leupold M3 scope and don't worry about getting the flattest trajectory. Trajectory doesn't matter if you can dial your scope reliably.
The .338/300Win will really shine with lighter bullets. Seems Barnes even offers 160's. A 160-300gr range in bullet availability is simply superb!
Nice thing about .338/300Win is it is a 15 minute job with a reamer, if you have a .338 Winchester already. If you want a .338-06 you have to buy one or do a custom barrel. Plenty of Nice Lapua, Norma and Nosler brass for .300Win out there, even some old Federal Match, floating around, I guess.
I suspect the WinMag case has more flexibility to offer than the 06 case, but opinions etc.
Best of luck with your gun