My Mark X started out as a 30/06 but was rebored to .35W with a 1:14 twist, and the barrel "crowned" at 23.6" and "throated" for 250gr (Speer) "pointed" bullets. My 7600 Rem. wears the standard 1:16 twist , 22" BBL and will shoot any of the loads that works in the Mark X just as well; all be it, a tad slower. I did shoot a Nilgai this year with the (obsolete) Nosler 225gr "clocking" 2700fps; and I had a complete "pass through" with good results. While the 7600 has as good a "lock up" (two lugs) as most "bolt" guns, it's extraction is suppose to be "weaker". Funny thing is that with all the 250gr loads I've tried (other powder included) recoil is enough to start the "extraction cycle" almost instantaneous after ignition, while not showing any pressure indications regarding the brass. (Yes, I know that without "pressure equipment" this statement is "suspect"; however, I'm not talking about loads at "max" or just under max, just 250gr loads in general!) I cannot load the 7600 with a long enough C.O.A.L to reach the "lead" without exceeding the length of the magazine, so even 250gr bullets have some "jump". As far as the new Speer manual and the listed 54.0gr of RL15 with a 250gr bullet, while listing two other loads in the same column above 2600fps, I would venture a guess they chose not to pursue a load beyond that the "manufacture" published. My "Lee Revised" manual lists the 59.5gr load as previously published by Alliant; while it is a given that Lee obtained permission to reproduce data from other manuals, I do not have the latest "revision to the revision" that is forthcoming. It will be interesting if any change is made to Lee to the "original" charge! I do understand that RL15 is more sensitive to "temperature" changes than the newer powders "hitting the market", but I have been entirely satisfied with RL15 in the Whelen as originally presented.
