A collimator is a simple device that uses a reflective surface and lenses to replicate a target at infinity.
I carry a lot of gear to the shooting range, so much, in fact, that I have to requisition my parent's minivan just to carry it all. Some of it is snivel gear, some of it superfluous, but there are a few items I simply cannot live without. A collimator is one of those have-to-have items for me, or any other shooter who likes to save time, trouble, and ammunition.
Ever fire a 100-yard sighting shot at an 11x17-inch target and completely miss it? We probably all have. After that, we're reduced to firing at the left edge, right edge, top, and bottom until we get a shot on paper in order to adjust the scope. The thought of wasting precious ammo keeps me up at night, and a collimator will get that first shot close to center almost every time. It also saves you the agony of trying to pick out the most recent bullet hole from the thousands of other holes in the target board around the just-missed target.
There are all kinds of collimators, which, by definition, is a device that narrows a beam of particles or waves. Oncologists use them to focus radiation waves onto cancer cells; industry uses collimators to focus lasers. When applied to the shooting world, collimators use a bit of optical trickery to replicate a distant target without introducing parallax to the equation. The device allows one to calibrate a riflescope so it is aligned with the axis of the bore. In short, a collimator creates a target 100 yards away using just the length of a rifle barrel, a reflective surface, and a few lenses, and it gets the scope and barrel pointed in the same direction without ever firing a shot.
Most collimators use an expanding or caliber-specific arbor--I've also heard it called a spud--that fits snuggly into the bore and onto which the collimator is affixed. Looking through the scope, a grid or reticle should greet the eye. It is just a matter of adjusting the windage and elevation so the scope's reticle aligns with the collimator's reticle or the center of the grid. The scope and the bore are then aligned, but keep in mind that the two axes are parallel or converge at some point that may or may not coincide with the bullet's trajectory exactly where needed. Quite a few factors, including the scope's height above the bore and miniscule alignment errors, prevent the bore sighting from being perfect every time.
You will, absolutely, positively, have to fine-tune the rifle so that the reticle and bullet's point of impact are at the same place. Working behind the gun counter at several gun stores, I always took a few minutes to bore sight a customer's rifle/scope combo and was amazed by the number of people who thought the gun was ready for the field. The aforementioned factors and the error induced by the shooter usually account for a couple of inches at 100 yards. My rifles are usually within 3 or so inches, but never have I been perfectly zeroed after bore sighting.
Care must be taken when inserting the collimator's metal arbors or spuds so as not to damage the barrel crown.
This rifle would probably not be on paper according to the collimator's grid; it needs 16 inches of elevation adjustment and 4 inches of windage.
There are other solutions to the problem, most notably centering a laser-emitting device in the bore and moving the scope's reticle to the laser's point of impact on a distant target. Some companies use dummy cartridges, others use an arbor with an attached laser. I have used both and found them to be very handy, but the system still must be double-checked like a collimator. The only downside is the necessity of a downrange target--most require at least 25 yards--and picking up the laser. That can be tough on bright, sunny days.
A collimator does not require a trip downrange, which is nice when the firing line is full and you have to time your trips downrange with everyone else. Both collimators and arbor/cartridge-mounted lasers are more accurate than the old tried-and-true method of peering down the bore, and they work on action types that do not allow a clear line of sight down the barrel.
Do not forget to toss a collimator or bore-sighting device in your luggage before the next big hunting expedition. It can be a huge help if the baggage apes happen to drop your rifle case from a great height and knock the scope around. Before your trip, attach a collimator to the zeroed rifle and make a note of where the crosshairs fall on the grid. In fact, make a note card and put it in your rifle case. Once in camp, attach the collimator and cross-check it with your notes. If the scope has been jarred off zero, that should be indicated on the grid. Move the reticle back to the correct position and fire a few shots to confirm your zero. Laser devices can obviously be used the same way, just be sure your marked target is exactly the same distance in camp as it was on the range.
Collimators and laser bore-sighting devices are relatively inexpensive. Caliber-specific cartridges usually run $50 to $70, and multi-caliber kits go for $40 on up. Leupold makes a pretty cool magnetic collimator that works on most every barrel; it runs $70. With premium ammo running $40 or $50 a box, it is pretty easy to see how a collimator can pay for itself in just a few range sessions. And on top of that, it will save some time and a little bit of your sanity.
Laser bore-sighting devices are pretty handy, but they require the use of a downrange target.