The OP stated is brother is "on the road for weeks at a time" (and it's his rifle!). Doesn't sound like he has much extra time to neck up cases, fire form 'um, and work up a load. The original question was about the 6.5x.284. Now, here we are at the .6.5x.257 Roberts Improved? KC, any of those mentioned, the 6.5x284, 6.5/06, 6.5x55 will work well on a 700LA, as would the 6.5x57. I just picked up my Hogdon manuel (the first one I grabbed) and flipped to the 6.5x55,,,,, 2857 w/a top load of RL-22, no practical difference. The skill of the driver (the shooter) would have more bearing than a few fps ever will!
in the original thread he stated they had a 700 chambered in 30-06. He was looking at the 6.5-284, and I said that they ate barrels. I didn't bother to state they also are well known for feeding problems, and case supply is always kinda iffey even on a good day. The 6.5-06 is a breeze to form as most folks know already. The 6.5AI is just as easy (neck down 7x57 brass, or neck up 6mm Remington brass, or even generic .257 brass). The reason I spoke of the 6.5AI is that it is a very good cartridge, and probably better for what his needs are than the 6.5-06 or the 6.5/284. It actually works better in the Remington magazine well better than the 6.5-06 when using very long bullets seated out near the lands. The gist of my post was that when you have a long action, then why not take advantage of it?
6.5x55 cases are not common in any place I shop. Yes they'll order it! I had a need for a couple hundred rounds a few weeks back, and three shops said they could order it in. I chose to use 6mm brass instead as I have about a thousand pieces that are virgin. Now I read the 2857fps post using RL22 and the 140 grain bullet (note: there was another 2800+ fps load in the Hogdon powders section). But lets be somewhat fair about this. The barrel used was a 28" barrel!! Now we drop back and look at the 6.5/.257 (same as a 6.5x57). The are showing 2732fps using H4831 powder (there's better) out of a 18" barrel!!! Interestingly the Accurate powder manual shows 2629fps with a 24" barrel and AA4350 powder. It's probably good for another 100fps even though they rate it as a max load. Speer shows 2671fps with a 22" barrel and RL22. Now digging out an old Hornaday book, they are showing 2500fps with an 18" barrel in the 6.5x55. Yet they also show the 6.5x57 with a 23.62" barrel at 2700 fps (both actions used were Mausers). Interestingly the #7 Hornaday manual shows 2600fps with a 29" barrel (I'll admit the load seems soft), and 2700fps with the 23 5/8th's barrel in the 6.5x57. Yet the 6.5/.284 shows the same 2700fps. and the 6.5-06 shows 2900fps with a 24" barrel.
Now all the rounds are fairly close to each other, with the exception of the .284 case and the 06 case as they should be. But then we look at the barrel lengths used, and we have once again got apples and oranges. Of course the shooter could simply go out and order a .270 Winchester barrel, and be done with it!