SCOPES: BACK TO ZERO
Created on 14th May 2009
VINCE BOTTOMLEY explains how to mount and zero a scope - and helps out a struggling shooter
YES, WE have covered this one before but I still receive emails on the subject and just recently I was prompted by an experience at my home range. I was playing with some test loads on our 100yd range but I couldn't help notice a shooter a few benches away who was busy firing round after round downrange. When he opened a second box of ammunition, I sauntered over.
"It's a new scope and I can't seem to get on." I invited him to shoot at his target while I watched the sand back-stop. The bullet impacted just to the right of his target! I could have told him to wind on about 15 minutes of left wind but the elevation was some way off as well. "Have you bore sighted it?" I enquired. My question was met with a quizzical stare.
The rifle was mounted on a bipod and after removing the bolt, I swung the rifle over a few degrees towards one of our flagpoles about 400yd away. Looking down the bore, I positioned the rifle so that the finial on the top of the flagpole was in the exact centre of the bore. My rear sandbag under the butt provided a firm support.
We should of course see the same image, the top of the flagpole, when we look through the scope. I could, just - but it was way over to one side and low. Being very careful not to disturb the rifle, I wound the scope-turrets to bring the crosshair onto the top of the flagpole. Another check through the bore to see we hadn't moved the rifle, a final tweak of the turrets and we are almost ready to shoot.
Almost? Let's think about what we have just done. Although the line of sight through the bore and the line of sight through the scope converge at the flagpole 400yd away, we'll assume for now that the two lines of sight are parallel. If we were now to take a shot at our target at 100yd and we have done our sighting work perfectly, the bullet would impact about 1.5" low, as this is the distance our scope is mounted above the barrel. So, we can wind up one and a half minutes of angle (MOA) to compensate for this. Also, we have the bullet's trajectory drop to consider and, as this is a 308, it will be a couple of inches at 100yd, so wind up another 2MOA.
I invited our shooter to take a shot at his target. The shot impacted about 3" away from the bull. He was amazed and reached for the turret to make a final zero adjustment. "Hold on - no need to guess." I set the butt of the rifle on my back-bag again and positioned it so that the crosshair was on his aim-point - the bull. However, although the scope was pointing at the bull, the barrel was clearly pointing elsewhere, judging by the position of the shot hole in the target. To get the two to coincide, we turned the turrets and wound the crosshair onto the actual bullet hole - no need for guess-work. The second shot went through the bull. That's all it takes - bore sight and two shots.
Forget collimators, lasers and the like - bore sighting is quick, easy and free! A few rules though:
1. You must support the rifle solidly, front and rear
2. Always choose a distant object like a telegraph pole, the further away the better for best results
3. Re-check your bore sighting a couple of times - it's almost impossible not to move the rifle when you twiddle the turrets
4. Remember to add the distance between your scope and bore - usually between 1.5" and 2"
5. Don't forget to consider the trajectory drop, which will of course depend on your cartridge and the distance you intend to zero at. If it's 100yd, 2" will do for most popular cartridges
I think you'll agree that was pretty simple, but let's go back a step or two. What about actually mounting the scope on your rifle?
Firstly, choose some good quality mounts. Look at the photograph. All these mounts are for the popular Weaver (or Picatinny) rail but choose carefully - good quality components pay off in the long run. Like everyone, I'm always on the look out for a bargain but play safe and invest in a pair of mounts made by a scope manufacturer and you won't go far wrong. I can recommend Leupold, Burris and Nightforce but there are many others. Avoid the vertically split ones shown in the centre of the photograph.
I would also recommend a good quality one-piece steel rail like the Farrell or the ones supplied by Nightforce or Badger Ordnance. If you shoot out to long distances, get one with a built-in taper - 20MOA is usually sufficient. Avoid two-piece mounts if you can. If you stick with the Picatinny system, you will be able to quickly interchange scopes between rifles.
Before you start to mount your scope rail, degrease the action surface and the underside of the rail and apply a thin smear of Araldite, then do up the screws lightly to just pull the rail down. It's hard to avoid getting glue on the threads so a bit of wax polish will help here. Clean off any excess glue with a cotton-bud. This will ensure perfect bedding of the rail onto the action. Let the Araldite set overnight before continuing. Remove the screws, apply a tiny smear of lube to the threads then nip them up tight. Remember, however, that these are tiny threads and will strip easily, so don't overdo it! Also, check that the front screws are not too long and impinging on the barrel-tenon threads. The holes in the receiver are usually drilled right through and if you screw into the threads and chew them up, it will make barrel removal very difficult and could result in a ruined action.
Now, mount your scope rings on the rail. For best support, place the rings as far apart as possible but ensure that you have enough leeway to slide the scope fore and aft a little for eye-relief adjustment. Nip up the mounts onto the base and remove the top clamps. Good quality components will ensure that any alignment errors are minimised but some may still be present, so the first job is to lightly lap-in our rings. This will also remove any ‘high-spots' which could ring-mark our scope tube. If there is a serious misalignment problem, it will be at least highlighted by lapping, if not eliminated. Wet & Dry paper (320 grade) wrapped around a suitably sized mandrel is required for the lapping. Remember to mask off the ejection port - we don't want any abrasive particles in there! It's not necessary to lap the top clamps but check for any obvious high spots or burrs and remove using the mandrel and Wet & Dry paper.
Once you are happy with the rings, we can drop in the scope. Please don't be tempted to wrap any sort of tape around the scope. We are mounting a 1" diameter tube in 1" diameter rings (or 30mm) and a turn of masking tape will swell the diameter by 10 thou. This means that when you tighten the mounts you will be creating pressure on the body-tube. Trust me, correctly aligned and lapped rings will not mark the scope and will not allow the scope to move under recoil. A metal-to-metal fit is the best engineering joint between two precision components. Although final alignment of the crosshairs (vertical/horizontal) will inevitably be done on-range, you can get it close by using a couple of cheap bubble levels - one on the scope-rail and one on the scope turret. Do up the screws on the ring clamps evenly, making sure the gaps at each side are near enough equal when finished, for a professional-looking job.
One thing we haven't mentioned is ring height. This is governed by two things - the scope's object bell must obviously clear the barrel so the larger the objective, the higher the rings. However, as long as your scope clears the barrel, I see no point in mounting it any higher than you need to. Secondly, if you have to use high rings, you may find that it's difficult to get a decent cheek-weld on the stock. If your stock doesn't have an adjustable comb, something like the Hunters of England ‘snipers cheekpiece' will solve that one. Personally, as a benchrest shooter, I avoid resting my cheek on the stock (except when shooting off-hand) so this is not an issue for me but I know some shooters prefer to ‘snug up' to their stock.
The final issue we need to address is eye relief. When you bring the rifle into your shoulder, whether you are prone off a bipod or standing offhand, having the correct eye relief will make that first shot and any subsequent follow-up shots so much quicker. Of course, we also need to consider recoil - get too close to that ocular and you could end up with a nasty cut eyebrow! So, if anything, you might find it's safest to stretch a little to obtain your eye relief. Get someone to measure how close you are to the scope - about 3" is average for eye relief and far enough away to prevent scope bite.
One more job before we are ready to shoot. We need to focus the crosshairs. Point the scope at a light-coloured plain background - a nice blank grey sky is ideal - and turn the eyepiece until the reticle is in sharp focus. Although you may re-focus the scope on your target at different distances, you should never need to re-focus the reticle. Finally, check for parallax. With the scope focused and pointing at your target and the rifle solidly supported fore and aft, peer at the target and move your head very slightly up and down. The crosshair should stay in position on the target. If the crosshair moves as you move your head, we need to slightly adjust the focus until we get rid of the parallax and the crosshair stays in position.
So, the next time you come to mount a new scope remember, 95% of the work can be done off range. You can mount the scope, bore sight, focus the reticle and adjust for eye relief anywhere but please, don't frighten the neighbours! When you get to the range, three shots maximum should have you zeroed.
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