Lot's of open space out here in Arizona, so your best shot may be at 500 yards plus on the next ridge over.
Does the 2.5-10 have enough magnification for those Coues Deer?
A clean kill on a trophy Coues buck at 500+yds under field conditions will not be easy. I know some diehard Coues hunters and don't know anyone with a kill past 350. That doesn't mean it can't be done, maybe they *chose* not to take those long shots. Just keep in mind that a good buck is hard to find and it is very unlikely that you will get a second shot. Stalk closer if you can. A trophy on the wall is better than a story of one you missed or wounded at 600 yards. I'm not much of a deer hunter and I've only shot one Coues buck, a nice one at 100 yards near Roosevelt Lake.
With that in mind, "enough magnification" is subjective and in part depends on your eyesight. Keep that 6.5-20x Mark 4 handy and look at animals at various powers and at various distances to figure out what you would be comfortable with. Also pay attention to field of view, ease of acquiring the target quickly and mirage.
I usually hunt with my scope set on 5x in case something pops up nearby. So a Leupold 6.5-20x, which tunnels considerably in the FFP versions that I have, would only be on a dedicated long range gun. On the other end, I don't think 2.5x is necessary for an AZ deer rifle. 4x would be fine.
As for max power for the 500 yard shot, I can offer my experiences. I shot an elk at 400 yards on 12x and a small mule deer (still bigger bodied than my Coues buck) at 550 yards on 16x. I could have used less but I had the mag available and it made for comfortable and precise aiming. I like picking a spot in the heart/lung area without feeling like my crosshairs are subtending the animal's entire chest.
So, if I had to offer suggestions, I'd say Premier Light Tactical 3-15x50 or Leupold Mark 6 3-18x44. I used to own the former and I like the Gen 2 mildot for hunting. For the latter, I'm a big fan of Leupold's TMR in FFP, although I have not actually seen it in the Mark 6 except in photos. Next on my list is the recently introduced 119214 version with compact M5C2 turrets and the illuminated TMR. But you can save a grand by getting the basic non-illum M5B2 TMR model.
Both of those scopes are nice and light. If the S&B 4-16x42 or 4-16x50 was lighter I'd suggest it as well, along with the Nightforce 3.5-15x50.
In summary, plenty of experienced shooters could kill a Coues deer at 500 yards with the Nightforce on 10x, but I've been hunting and shooting for close to 30 years and it wouldn't be my first choice... You have to know what you're comfortable with. I envision my 6.8 SPC with the NF 2.5-10x42 as a 300yd hunting gun and 500yd steel gun. For my uses and preferences, the rifle, cartridge and scope are well matched.
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