So, you have a 308. It is a fine 300 yd Elk gun. You need to know how you shoot prone, over your backpack, etc to know how far you can shoot.
Any guide worth his pay can get you within 250 yards of a trophy bull. More likely 100. They are not magical people, they just chase the same Elk everyday across the same piece of land. A good guide should know where the elk are going before they do.
I hope this guide is not seriously expecting clients to shoot 500 yd elk. That is what commoners like I get because they have been chased all day on private land before the show themselves 600 yards from me! Guides get paid to guide, not point out unshootable elk!
Back to real elk rifles. There are 2 kinds....
Point and shoot....these are guns you sight in at 250 - 300 yds and just aim center of vitals to 400 and top of elk at 500 with practice. They are 300 win mag, 270 WSM, 7mm STW, 300 weatherby, etc. they all are big recoil hard to shoot guns and i find when i put a paper plate at 400 yards, their owners miss it!
Next, are the big enough to kill, small enough to shoot...these are 308, 30'06, 7mm Rem Mag, 338 Federal, 270 Win, 280 Rem, etc. these too with the right bullets kill Elk out to 500 yards, If you can tape a range card on your arm, learn to range(or laser range elk) and learn tricks about scope adjustment or hold over. You need to practice a bit more, read the wind a little better and quit trying to wing(as in wing and a prayer) shooting elk at 500 yards.
Just to set the record straight, I shoot a Savage 12BVSS in 300 WSM at all big game. It has a Burris XTR 3-12 with Mildots and tactical turrets. It weighs 14.5 lbs and shoots fine off my backpack. Prone 300 yd hunting groups are a breeze. It is amazing what a couple of pounds does to reduce recoil and solidify your rest. It was designed as a 600 yd elk gun. All I need now is 600 yds to practice at!