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Rocky’s Reloading Room |
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Bore Sighters: Rifleman's Secret
There's an inexpensive item of shooting equipment that can save shooters money, allow them to shoot more accurately and even help diagnose problems with their rifle. Yet, inexplicably, it's also one of the least understood and rarely owned bits of shooting gear despite its moderate cost. It's a collimator, also known as a bore sighter. Bore sighting is a process which roughly aligns a gun's sights with its bore. That may seem elementary, but it's important to establish what bore sighting does as well as what it doesn't do. Because I work in a gun store in addition to my writing for Varmint Hunter, I often mount upwards of a hundred scopes each year. Every single one gets aligned with a bore sighter. And yet, it seems that very few customers clearly grasp what the process entails or what it does for them. Many believe that bore sighting is the same as sighting in. In other words, that after bore sighting, they can immediately go hunting. It isn't true. Others believe that a bore sighted gun has its barrel and sights perfectly aligned. That isn't always true either. Still others think that if you check a sighted-in gun with a bore sighter, the crosshairs will be centered in the bore sighter's grid. That still isn't true. If you share any of those misbeliefs, read on. If not, you get one gold star from the teacher. But read on anyway. To begin, let's examine why none of the beliefs above are true. Later, we'll see what is true about bore sighters and why they are so useful. First the myth that a bore sighted gun is sighted in. Bore sighting is a static adjustment. That is, it roughly aligns the vertical and horizontal crosshairs or scope's sight line with the vertical and horizontal bore line when both are at rest. It does not take into account pressure on the barrel by the stock, the shooting technique of the individual shooter, movement of the gun or vibration of the barrel when fired, or the ballistics of the bullet and load being used. After careful bore sighting, everything might be aligned pretty well while the gun is sitting still. But the instant you fire a cartridge, all bets are off. Especially a bet on where the bullet will strike. Why do I say roughly aligned? Aren't scopes and bore sighters highly precision instruments? Individually they may be, but used together that precision diminishes. Mount a small diameter scope in low mounts on a rifle with a small diameter receiver and your bore sighter might tell you one thing. But a huskier rifle receiver wearing higher mounts for a large-objective lens scope alters the geometry of the whole setup. Think of the scope as a rear sight and the collimator device as a front sight and you'll grasp what I mean. Any change in the position of either sight will have a radical effect on where the bullet hits out yonder. All of this partially explains why a bore sighted gun is only roughly aligned and also why such a gun is most emphatically not sighted in ready for the hunt. So what the heck good is it? Simple. Bore sighting is much, much more accurate than simply bolting a scope to a gun and heading for the range. Have you ever mounted a scope by the "bolt it on and load 'er up" method, I'd wager you've also had the experience of firing a box or more of ammo without even hitting a target at 100 yards. Or of being nearly off the paper even at 25 yards. Or of running out of scope adjustment before your shots got where you wanted them. Or perhaps your rifle was perfectly sighted in with your favorite load, but you changed to a new scope. You had to repeat the whole sighting in process again, using valuable ammo in the process. Or you have several loads, perhaps one for varmints and another for big game. How do you change from one to the other reliably without yet another trip to the range? All these situations are made easier, without lots of shooting, with a bore sighter. HOW IT'S DONE When I mount a scope on a rifle, here's what I do: First, as with every other activity around guns, unload the rifle, then double check the chamber is empty. After inspecting, degreasing and mounting the bases, I'll install the rings and roughly center them left and right. Next, I remove the ring top halves and simply lay the scope in place to make sure the rings and scope aren't askew. I also check that the scope's forward bell clears the barrel. Next, I'll lightly install the top ring halves with the degreased screws left loose. Now I check that the bolt handle or hammer clears the rear scope bell when operated normally. Then I'll check for proper eye relief by closing my eyes, mounting the rifle then opening my eyes. A black ring in the scope's field of view means the scope needs to be adjusted forward or back to get optimum eye relief. Finally, I'll align the vertical crosshair either with a separate tool or by sighting through the scope with my eye at the buttplate. Now I'll carefully snug the ring screws down before rechecking clearances front and back, eye relief and vertical alignment. (You can do even more to properly mount a scope. Lapping rings and checking ring alignment are just two such procedures.) Only now can I start bore sighting. The first step in bore sighting is to center the reticle within the scope. Most new scopes come from the factory centered, but any other scope should be reset. Gently turn either adjustment dial all the way to one stop. For example, turn the elevation knob all the way towards UP. Now turn it all the way to DOWN, carefully counting how many times the knob goes around from one stop to the other. Don't force the adjustment at either end, and do keep an accurate count of turns. Let's say that the knob goes around four and a half times from UP to DOWN. Now turn the knob back towards UP two and a quarter turns, or exactly half the total movement available. Repeat with the windage knob and your scope's reticle should be fairly well centered. The reason for this step is to insure that you don't align the scope off axis with little or no adjustment left. Now prepare the bore sighter. I'll assume you are using one of the more common optical types by Bushnell, Redfield, Tasco or others. Thoroughly read the instructions for your unit. Most types have one or more sized studs to insert into the bore at the muzzle, and an optical lens and grid collimator device. Select the proper stud for the caliber of the rifle, attach it to the collimator and insert and tighten the stud into the rifle bore with the collimator upwards. Lay the rifle in a padded support so you'll have both hands free while you perform the rest of the procedure. Look through the rifle scope and you'll see the crosshairs superimposed on a square grid pattern. Gently twist the collimator if necessary to get the grid and the vertical crosshair aligned. Adjust the scope's power setting, if it's a variable, so that the grid fills most of the field of view. OK, here's where it might get a bit confusing, so think about this a second. The crosshairs show you where the scope is pointed. The grid shows you where the barrel is pointed. If the center of the grid is sitting above and left of the crosshairs, the barrel is pointed high and left. Scope adjustments are labelled to change bullet impact (barrel), so to center the grid, you'd turn the knobs towards DOWN and RIGHT. If you have windage correction scope bases, use them first to get the vertical crosshair and grid centerline as close together as possible, then tighten the windage screws firmly. Now use the scope's windage knob to fine tune the setting. That's the easy part. Align the horizontal crosshair with the grid by using the scope's elevation adjustment. Again, think of it as moving the grid. If the grid is high, turn the scope knob towards DOWN until everything is centered. If you're using a gun and scope combination that puts the scope's centerline 1.5 inches above the center of the rifle's bore, you are almost done. But if your scope is either higher or lower than 1.5 inches above the bore, things can get a bit tricky. The reason is that the collimator is designed around that 1.5-inch "standard" scope to bore distance. As discussed earlier, anything else is like raising or lowering the rear sight on an iron-sighted gun. To compensate, you simply adjust the crosshairs slightly above or below the grid's center. If your scope is mounted much lower than 1.5 inches above the bore, set the crosshairs just below the grid's center line. If your scope is taller, set the crosshairs slightly above grid center. Once more, a little thought and this will make sense. Most collimator grids have a scale notation somewhere in the field of view. My Bushnell unit says that one grid line equals four inches at 100 yards. For high or extra high scope rings, I've found that one quarter line above grid center is about right. For those fortunately out of style see-through tunnel mount types, it might be one half grid line. If you run out of vertical adjustment before you get centered in the collimator, you may have to shim either the front or rear scope base to obtain more adjustment. Here, think of the scope bases themselves as iron sights. If you can't get enough UP, shim the rear base upwards. If you run out of DOWN, shim the front base upwards. Most gunsmiths carry shims of varying thicknesses and properly sized to match most scope base screw distances. Get the right ones and don't try to cobble something up. Now the most important step. Final tighten all screws, then look through the collimator one last time. Nothing should have moved. If anything has, you may have mis-mounted the scope bases, rings or the scope itself. Any misalignment of any item can create torque or stress in the scope. But it's better to learn it now than after burning several boxes of ammo in a gun that won't sight in or hold its zero. Better to pull everything off and start over than suffer a rifle that's out of kilter. Worse, the recoil of shooting can destroy a scope that's mounted poorly and in mechanical stress to start with. Now to the range. Take the bore sighter with you. Begin at 25 yards like your momma taught you. Fire one shot and adjust the scope until you are hitting dead on. Remember that you'll have to multiply the scope increments by four at 25 yards to get the desired amount of bullet movement. (To move impact one inch at 25 yards with a scope that has quarter inch clicks, you dial in 16 clicks.) It shouldn't take more than two or three shots. Then set up a fresh target at 100 yards. Fire a three shot group. Measure the distance from the center of the group to your desired impact point and adjust the scope. Fire three more to verify and you should be finished. Now with the gun absolutely positively and without doubt completely unloaded (remove the bolt to be damned sure), reinsert the bore sighter and note where the crosshairs and grid align. Write down that setting. For example, crosshairs at one grid line above and one-half line right of center. That's the sighted-in setting for that load in that rifle with you shooting. Hardly ever will you still have the crosshairs and the grid still perfectly centered. Again, as we've already discussed, barrel vibrations, your personal shooting style, and many other factors will make a gun shoot to a different impact point than it points to at rest. I've had some extremely accurate guns perfectly centered on target that had collimator settings as much as eight inches (two grid lines) off center at rest. Test some of your guns that are already sighted in and you'll see what I mean. While you're at it, write down those settings for future reference. Make sure you note the particular gun/scope combo as well as the exact load used to sight in. The more information you record, the better. OTHER LOADS, OTHER USES If you wish to sight in for other loads, go ahead and fire them, resetting the scope as necessary. After each load is sighted in, reinsert the collimator and write down the setting for each. Now at any time, if you want to change loads or even change to another scope, simply adjust the scope to the recorded settings and you'll be very close to perfectly sighted in. One or two shots at most can be fired to verify. Most of the time, you'll be surprised at how close you'll be. Want to recheck your zero after travel to your hunt camp or a nasty bump to your gun? Drop in the collimator, a quick glance through the scope and you're done with no shots fired. By the way, it's not common knowledge, and the information isn't even printed in most bore sighter instructions, but you can also align the collimator itself. On the Bushnell unit, the front trim ring is removable. Under it are four tiny screws located 90 degrees apart. They work just like scope adjustment knobs. Have a load in a rifle that you'd like to use as your baseline? Put the collimator in place and gently adjust it until the grid is perfectly centered on the rifle crosshairs. Think of it as reverse bore sighting. Now you have the collimator adjusted to the rifle. This is also valuable knowledge if you ever drop the collimator and jar it out of alignment. Simply reset it until it and a known scope setting are aligned and you're back in the bore sighting business. OK, by this point you may agree that a bore sighter is a darned nifty piece of gear to have. But there are still more uses for one. You can use it to check stock bedding, for example. Install the collimator and note the setting as closely as possible. Now loosen the front action screw slightly. If you notice any change in the setting with a freefloated barrel, there is some unwanted "spring" in the stock. Repeat with the rear action screw. Generally, vertical movement signals a pressure point under the action bedding somewhere, while a horizontal change usually means pressure along the sides. If your barrel is supposed to have up pressure from the fore end tip, use your bore sighter to confirm the correct amount of pressure. Install the collimator with the front action screw slightly loose, then slowly tighten it until the bore sighter reading is the same as when properly sighted in. Neat. All the above refers to rifles, but a bore sighter is equally useful on handguns. Treat single shots like the Thompson Center Contender or Remington XP-100 as rifles. You may notice that sighted-in settings, however, show the crosshairs considerably above and left of the grid center. This due to the much greater amount of barrel rise and torque of the calibers used in these guns. The at-rest barrel must usually point a goodly amount below and to the right of the sight line to compensate for recoil and bullet torque at firing. On handguns with low-power scopes, it can sometimes be difficult to see the small grid lines clearly, but by simply quartering the grid with the reticle, you can still do a good job. It's also possible but difficult to use a collimator to verify iron sights on any firearm. Very low iron sights may align with the lower body of the collimator, and don't give a complete picture of the grid. But you may be able to mentally project the bottom part of the grid lines and do a fair job. That difficulty aside, a good bore sighter is a most valuable tool, one that I don't think many shooters would be without if they started using one. It's not a do-everything tool, and it may not do what some shooters think it does. But it certainly does a heck of a lot more than it doesn't.
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