SR,
Why not use your .300WSM and take all of the guess work out of it? but if you just really want to use your 6.5 it'll do the job if you do your part. Just be careful to pick your shots and watch your angles.
BTW.
If you use a good bullet there is no .338 Lapua that will stop in one little ole milk jug full of water.
Any of the premium bullets will penetrate end for end on an elk at close range even out of a 3000 FPS + rifle. Your guide must be using crap bullets in his .300. I can promise you that a 180gr X will go all the way through any elk on this planet at 20 yards even when pushed at 3000FPS+. If the outfitter wants to use two rifles that is fine but with decent bullets either will do either job might handily. I am very leery of these kinds of stories. I can’t think of a elk hunting adventure either guiding or hunting ion my own in the last 20 years that I haven’t hunted both timber and long range parks and canyons in the same hunt usually in the same day. Having to go back to the truck and get my timber rifle or my long range rig just doesn’t work in reality.
I like the .308 calibers at 180 grs and up but I've killed elk with smaller. The first elk I ever killed was with a 140 gr bullet out of a .270.
Some of my favorite elk rounds are below. All of these rounds are good long range and make fine timber rigs as well.
.308 win 180 gr bullet @ 2600
.30-06 180gr bullet @ 2700
.300 mag (your choice) 200gr bullet @ 2800
.338 win 250 Gr bullet@ 2700
.375H&H 270gr bullet @ 2600
I don't like 7MM just a personal preference thing. It's nothing more than an belted necked down .30-06. I just use the real thing the first time.
It does just fine on elk.
PS
If I ever had a guide, any guide tell me that I needed a 10X scope for anything I'd get my money back right away. What a DORK. I worry about the low end a hell of a lot more than the high end of the magnification. Of course I don't lob long range artillery at elk either.
I meet to many of these long range heroes out here in Co. I like to take them to the range and put them on the six hundred yard bench and let them whack away at a 12" steel plate. I've yet to see one of these heroes ring it in the first three shots. And that is from a bench on a known exact range under controlled conditions. With a super mag and a lunar observation scope on top. Yet 20 minutes before they all had their hairy 700 yard elk shooting story, cross canyon in a driving snow storm of course.
It really rankles them when I pull out a 19" Steyr Scout with a 2.5X scope on it in .308 win and ring it with all five shots. Of course what they don't know is that I've figured out a long time ago that the top of the frame on that the steel hangs from is just the right hold over for my little .308, about 56 inches high all I need to do is rest the horizontal wire on the top edge of the frame and if there is no wind I can ring it every time.
I wouldn't try that on a critter under field conditions. But it sure deflates some of the ultra mag egos out there.