THIS is the time to make sure that your deer rifle shoots where you aim it, a process called zeroing in.

Zeroing in involves adjusting the sights, whether open, peep or telescopic, of your gun so that it delivers the projectile to the point of aim at a known distance. What that distance should be is determined by the ballistics of the cartridge you are using and the game you are hunting. When shooting woodchucks in the back meadow, you would probably want to zero in your rifle so that the projectile was never more than two inches above or below the point of aim out to the maximum range; with an animal the size of a deer, the maximum up and down deviation could be at least 4 inches.

Because the .30/30 is the classic whitetail deer cartridge, it can be used as an example of how to decide on the point of aim range. Zeroed in to hit the point of aim at 50 yards, a 170-grain bullet from a .30/30 would be about three-quarters of an inch low at 100 yards, a little more than 4 inches low at 150 yards and nearly a foot low at 200 yards. This would limit you to ranges of 150 yards or less. Zeroing in the same cartridge to have the bullet hit the point of aim at 175 yards, would put the projectile slightly over 2 inches high at 50 yards, less than 3 1/2 inches high at 100 yards, less than 2 inches high at 150 yards and less than 3 inches low at 200 yards. In the heavily forested areas of the Northeast, most deer are shot at ranges of less than 100 yards, but every once in a while you'll encounter a situation when a deer will be walking through open woods 175 or 200 yards away. When that happens, it's helpful to be able to hold right on the animal rather than a foot above its shoulder.

You should test fire your rifle each year. If it has fixed, open sights, the chances are remote that anything will have changed, but peep sights and telescopic sights are more vulnerable to being jarred out of alignment. Fire three rounds from a rest. If you have to make corrections, remember that each click on the windage and elevation knobs on most American-made peep sights alters the bullet's point of impact a quarter of an inch at 100 yards. The same is true of many scopes, although their adjustments might involve only a calibrated scale without clicks. If you don't have the manufacturer's literature that came with your sight, you'll have to experiment.

If your rifle shoots way off the mark, there is - if it has a telescopic sight - a quick way of correcting most of the error. Using sand bag rests, squeeze off one shot. If you are satisfied that you were right on the bull's-eye when you touched that round off, aim at the bull's-eye again, anchoring the rifle in position with the aid of the sand bags, and alter the scope's adjustments, moving the crosshairs left and right or up and down on the target until they are centered over the bullet hole. If the rifle hasn't moved during this maneuver - I sometimes have a companion make the scope adjustments while I hold the rifle with both hands - your next shot should be right on the money or so close that only minor additional corrections will be needed.

Open iron sights vary in design. The typical rear open sight has a notched ramp for elevation and you pick the notch that suits your purpose. Raising the sight raises the bullet's point of impact. Right and left, or windage, adjustments are another matter. Sometimes both front and rear sights are mounted in a dovetail slot, sometimes only the rear sight is so affixed and adjustments must be confined to it. Rest the rifle on its side on a piece of folded cloth or carpeting, and - using a mallet and a brass drift pin - tap the dovetailed base of the rear sight in the direction you want the bullet's impact point to move. (The reverse would be true of the front sight.) Most of the time, you won't have to move the sight more than a hair one way or the other. At 100 yards - if the front and rear sights are about 20 inches apart - the distance the rear sight is moved is multiplied 180 times at the target. To put it another way, if you tapped the rear sight 1/32 of an inch to the left, the bullet's point of impact would -at 100 yards - shift 5.62 inches in that direction.

It is most unlikely, but you may have to move the rear sight so far to one side that it won't hold firmly in its dovetail. One way to deal with that is to solder the dovetail in place. This means removing the bluing to get the solder to stick and re-bluing after the job is completed. This is not aesthetically pleasing - I've done it to two of my own rifles - but firearms whose sights are so far out of whack are not usually things of beauty to begin with.

Shotguns with special barrels for rifled slugs are available for deer hunting and some of those, particularly the ones with rifled barrels, provide excellent accuracy. Some have adjustable sights or telescopic sights and they are zeroed in just as a rifle is. When shooting slugs with an ordinary shotgun which has no sights, one can only take it to the range, observe whether it shoots right or left, high or low, at 50 or 60 yards and make adjustments for its idiosyncrasies when actually hunting.

Zeroing in can be a compromise. A short while ago I could not resist purchasing a second-hand combination gun, a Savage 24S-E with a .22 Winchester rimfire magnum rifle barrel mounted over a .410 shotgun barrel. I doubt that its previous owner bagged much game with the rifled barrel, which, when I fired it, shot 14 inches to the right at 50 yards. After tapping the rear sight into position and soldering it in place, I fired a few test rounds through the .410 barrel. At 35 yards, the center of the pellet charge from it is 8 inches to the left of the line of sight. There's nothing that I can do about those misaligned barrels. I just have to remember, when shooting the .410, to hold to the right.

Hunters who live in rural areas usually have access to a sand or gravel pit where they can zero in their rifles. Most city dwellers have to locate a rifle range. The big-bore rifle ranges at Westchester County's Blue Mountain Sportsman Center are open to the public Thursday through Sunday this month and next. For more information, call (914) 737-7450. The National Shooting Sports Foundation, 555 Danbury Rd., Wilton, Conn. 06897, sells, for $2, a directory of more than 900 shooting ranges nationwide that are available to the public all or part of the time.