...my nephew in Seattle uses on of these, training for x-country/ endurance @ the local ski area (5000')...
http://www.roguefitness.com/elevation-training-mask-2-0
...well being 'old school, shoot the biggest bullet you can shoot well' & a firm believer in hydrostatic shock, I'd see if I could get him shooting the .325 WSM w/ a 220gr. Sierra SBT (B.C. 0.521) over 63grs. of RL-17 @2800fps., ballistics wise, right up dere w/ the 8mm RM or .338 WM 210-225gr...
...was me, I'd skip the poly liner socks, & go silk or wool, SmartWool & IceBreaker both make a good, lite, tight weave merino wool liner. Since yer already getting into pretty good shape, you might want to consider a breathing mask to get you ready for the elevation, even the low spots are 7K...
...ballistically speaking, 600yds. & yer getting into the lower end of the NBT's performance window & 1K energy realm. Real life, 400yds., if you practice a lot @ 400yds. shouldn't be a problem for the 120gr. NBT on deer sized game...
...I've used the .25/ 120gr., 6.5/130gr. on deer & antelope, the 7mm/160gr. & .30/ 165gr. Sierra HPBT GameKings on deer & elk, consistently accurate, might not be the highest B.C., but you can't argue the effectiveness of their terminal ballistics...
...hmmm, appears I'm the "odd man out". Only rifle I have 'one' load for is the .325 WSM, the .530 B.C. 220gr. Sierra SBT @2800fps takes it from the 'ho-hum', mediocre class of most of the 200gr (.400 B.C.) 'book loads' to very comparable to the 8mm RM/ .338 loads. Everything else, even the...
...the design concepts are different. While a hunting bullet usually includes accuracy & B.C. in it's design the core is 'consistent terminal performance', expansion, penetration. The features that make a good 'hunting' bullet, like tapered jackets, etc., aren't generally used in 'match'...
...there's a good article on the ".257 Wildcat" (.25 Creedmoor/ .250-3000 AI 30* shoulder) in the Apr. '17 "Handloader" magazine. Just need a bushing die to set the neck, no fireforming, lets you use good Creedmoor brass for higher pressures (62K)...
...per John Barsness/ Handloader 4/17; "After my article "Reduced Rifle Loads" appeared in Handloader No. 306 (Feb 2017), Hodgdon contacted me w/ new info. After considerable testing they recommend against using IMR-4895 for reduced loads, suggesting only H-4895 & Trail Boss"...
...since you're applying the same amount of heat to the same amount of surface area in the bore, but increasing the outside surface area by fluting, it shouldn't 'heat any faster, but it should dissipate the heat quicker...
...the 26" barrel on my Creedmore is fluted, & there is one real advantage since it's still a "hunting rifle", the fluting allowed a med. contour that still balances @ the center of the action, making it fairly comfortable to carry all day...
...I can buy up to 3 elk tags, but usually only get 2, as the areas for an extra cow are over 100mi. round trip, instead of being able to see 'em from the front porch...
...haven't used the 140's, but I've used the 7mm 160's & .30 165's on mule deer & elk, the 6.5mm 130's on mule deer, & the .25 120's om mule deer & antelope, all w/ very good, consistent results...
...I'm going to repeat what Dr. Mike said, "Look @ the .325 WSM". I prefer the 220gr. Sierra SBT @ 2800fps (Winchester 220gr. 'factory' is rated @ 2840fps), putting it right on the heels of the .338WM, w/ about 20% less felt recoil & 30% less muzzle blast. Most of the data is fairly 'mild', but...