So, to make sure I'm understanding correctly, here's my recap...
FIL, so you don't want to piss him off too much.
He's stubborn, like most of us are.
His spot is a good one.
Elk isn't dying soon enough, which makes it a PITA for you to retrieve.
He's using enough gun (.30/06) but pretty terrible ammo (CoreLokt).
Here's what I would do, if it were me. My advice is free, so it's worth what you paid for it. I do wish you the best of luck, and hope you let us know how it works out.
- I didn't see anywhere where you said what optics he's using. You might consider an upgrade to a larger/more powerful scope to help make 300yds seem closer, and help with accuracy.
- I would sight him in with a 300yd zero, if you're not already, which it sounds like you are.
- If you can find them, I'd use 200gr Accubonds for this job. 180gr Accubonds would be my next choice, followed by 180gr Barnes TSX.
- Since he's not willing to move his spot, I would do everything I could to help his accuracy from his spot. That means setting him up with a good solid rest, maybe even a bench-type setup, depending on how much work it would be to do it.
- I'd make him practice. A lot. Using the same setup you're going to put in his hunting spot. If he won't do that, then I wouldn't take him, period. It's not fair to the animal if we don't do our part. Hang a 10" steel plate at 300yds and make him ping it.
- There is some value in the idea of using more gun, since this application isn't far from the limits of the .30/06. The .325 WSM would be my next choice, followed by the .338 WM, though I do favor short-actions over belted magnums. Only you and he can determine if upgrading is worth it. If upgrading, I would go bigger than a .30" bore size.
- The winch/cable idea mentioned above is a very good one if all else fails. A couple of pulleys and a few hundred feet of 3/16" cable would be worth it, especially if you could carry enough cable to use with an ATV winch. You only get 50ft or so of pulling with the ATV winch, but you can use something
like this to help out. You hook your winch cable into the loop, and grip the extension cable with the grab, winch in 50 ft, spool out the winch while sliding the cable grab down the extension cable, and repeat.
Again, best of luck, hope this helps.