The wise hunter makes sure his rifle is still shooting straight.
If possible, check zero the way you hope to shoot game. Changing the type of rest can change your point of impact.
You've traveled far. Airports swallowed your rifle case, then disgorged it with fresh dents and scratches. For the last dozen miles, your rifle has ridden tight in an age-hardened leather scabbard. Tomorrow, you will hunt. But will your rifle hit where you point it?
Checking zero in the field makes sense even if you've not dropped your rifle or had it extensively banged about during travel. Modern scopes and mounts will endure terrific punishment, but a point-of-impact surprise can also result from changes in bedding. If you zeroed in a dry place and are hunting in rain, tight bedding in a walnut stock can pressure the barrel as the wood swells, throwing your shot wide.
Swings in temperature can have an effect too, even with synthetic stocks. Ammunition temperature influences trajectory. And if you must borrow ammo or change loads, you'll want to see where those bullets strike. Also, you may find that the hunting environment affords you little chance to employ a rest or a bipod--the support you used to zero. Shooting from sticks or with a tight sling, you'll likely find a shift in point of impact.
So even if there's no reason to suspect your rifle has suffered a blow, checking zero in the field can improve your chances for a good hit.
The first order of business is to bring some extra cartridges for the task. You'll also want proper targets, because shooting at a can or a rock doesn't tell you enough about bullet placement relative to point of aim. Pack a half-dozen of the paper targets you like best, and a stapler or tape to affix them to a backer.
When you field-check zero, hold the rifle firmly, as if you were shooting at game. Use a sling or a bipod if you expect to use either on the hunt. Even if you have the luxury of a bench, apply the pressures and tensions on the rifle that you most often apply when shooting at game.
Check the placement of the first round from a clean, cold barrel. Firing three shots into a group without marking each one can leave you wondering if that flier was the first shot or the last. Commonly the first round will land outside group center. That's all right, as long as you know it.
Zero for the first shot; it matters most. Let the barrel get cold between groups if you find a warm tube changes bullet impact.
Zero so the midrange trajectory, or the highest point of the bullet's arc above sightline, is no more than three inches. I prefer bullets from most flat-shooting hunting rifles to land two inches high at 100. I can then aim dead on to 200 or even 250, depending on the load.
Don't adjust your scope unless you must. Hunting camp is no place to tweak and tune. A softpoint that lands half an inch left of your aiming point will still kill. On the other hand, shoot enough to establish an average. A single bullet may delight or dismay; neither reaction is warranted. Fire three shots at least.
Avoid an audience. You don't need advice, applause or criticism. Do not allow another shooter to fire your rifle as part of a zero check. You can't know if he sees the same sight picture as you, and you won't know if a bad shot is the sights or the shooter--or whether a center hit is an accident or proof that your zero is where it should be.
Don't put your ego on the line. Once, when checking my rifle after another member of the hunting party, I heard him say "Well, five inches high at 100. That's about right." He obviously wasn't sure about that, but I got the feeling he didn't want to let his uncertainty show. Subsequently, he missed two bull elk.
Be thorough. If you have to make an adjustment, test it by shooting. Don't assume that a handful of quarter-minute clicks will do what it's supposed to. Moving the dial can fail to produce any change in impact; it may also shove the bullet two or three times as far as you expect.
Once, to ensure my scope was properly zeroed, I fired 16 of the 20 cartridges I had brought with me. And there was no place within 500 miles to find more. My companions roundly criticized my profligate use of that ammo. But I killed three animals--as many as the trip allowed--with the remaining four rounds.
After you've confirmed or reestablished your zero, keep the rifle away from livestock and other hunters. Store it in a case or on a duffle bag under your cot. If you lean a rifle upright, it will eventually fall over. And you don't want to have to check zero a second time with the last magazine of cartridges in your duffel.