Variable zoom scope calibration

S

Silent Shadow

New member
Ok, this is inteded for air rifles, but I'm sure it's not too different that regular rifles. Only a matter of power and range I would think.


Let's say you have a 8-32x50 Swift scope on an air rifle (which is what I'm considering).

Then lets say you place targets (paper targets or whatever, that show you how far off you are from where you are aiming at) at 20, 40 and 60 yards.

Now the first shot you don't know what the heck will happen. You set your power to the minium 8x and see the target at 20 meters big and clear in your sights.

You shoot, and the pellet ends up low and to the right. So you adjust the clicks and shoot again, and do it until you nail that thing straight in the center. Once you get that, you can say you have zerod your air rifle to 20m right?

Now you want to do the same for the 40m target.

Now here's the catch. Will changing the zoom on your scope affect everything, or will it not matter?

Here's what I mean, let's say that you keep your 8x zoom and it's much harder to see the target that is twice the distance from your first one.

You estimate couple clicks down/up and shoot. Your pellet hits the corner of your target at 40m. Now you change the clicks on the scope and shoot until you get to the center of that target (making sure you know how many clicks from the zero it is in each instance). Once you nail the 40m target, you note the clicks from the 20m zero. You take your "range card" and note 20m "zero", 40m "8 clicks down" or whatever. Then you do the same for the 60m target, and note "16 clicks down" or whatever.

Then you have this nice rangefinding ability of your Swift scope. So you place a can at 20m, and you adjust the objective into focus, and then mark on the bell the 20m. Do the same for the 40 and 60m.

So now you have a nice rangecard with 20m intervals zerod at 20m up to 60m and you also know how to rangefind.

So you are all happy and now are confident that "ignoring wind" you will be able to hit anything at 20,40 or 60m.

So now you place cans at 20, 40 and 60m. You have your air rifle zerod at 20m.

Let's say you are deprived of the ability to estimate range and you don't know which can is at what distance. ;)

Now you look at one can, then focus on it with the 8x zoom as usual, and you see on the bell that you marked it as 40m. You are like "cool, no wind, so all I do is adjust the elevation and I'm set!" you take a look at your rangecard since you have the most horrible memory in the world ;) and you see that 40m is "8 clicks down". So you adjust your 8 clicks, aim at the can 40m away...squeeze and...BAM the can is hit...

Now that's all nice and pretty, and i don't know if any of it is right, but my question is here.

Let's say that you want to hit the can at 60m ok? Now you use the bell of the scope to tell you it's 60m (because you can't estimate remember ;)) and you look at your card and it's 16 clicks up, and you dial it in...and you are set.

BUT, what you now notice is that the target is 3 times the distance of your first can, so you can barely see the damn thing ;), so what you do is you use the full potential of your 8-32x scope, and you power it up to max zoom and focus again. So now the can in in your face, and you can easily notice the little drop that is sliding down the side ;)

So now...my question is, will you still hit the can? and...will the distance you marked on the bell still be 60m?

Silent Shadow
 
Totall Guess here

But I would say if you have a quality scope, then the zero will remain constant throughout the zoom range.

If you have a reticle, it should adjust with the zoom, keeping range estimation correct as well.

I'm interested in other's replies.

-LevelHead-
 
The literature for air rifles sez that the recoil is different, and scopes on "regular" rifles will break up internally under the different type of loading. From that warning, I'd assume your idea of a Swift scope ain't the way to go.

One clue as to the quality of a scope: Shoot at your target, and then change the magnification. The next shot should hit within the inherent accuracy of your rifle, be it air or gunpowder.

For instance, with my Bushmaster I fired a three-shot, half-inch group at 24X. I adjusted the scope to 6X and fired one shot; went back up to 24X and fired another shot--a half-inch from the 6X hole. Ergo, good scope.

I guess that in ranges from 20 to 60 yards you'd maybe have to adjust the objective lens for focus, but that's outside my experience.

Knowing the number of clicks to raise your impact point at different distances is a Good Thing--IF the scope's adjustments are reliably consistent. Not all of them are. However, without some sort of rangefinding dots or extra crosshairs to give spacings at different ranges, just changing the magnification won't tell you distance.

Hope all this helps,

Art
 
So in a few words, the variation of zoom on the scope should not make a difference on rangefinding (by adjusting the objective and focusing on an object) and on the zero?
 
Shouldn't, SS.

Some scopes adjustable objectives are sensitive enough to do a fair job of ranging through clarity/focus.

Too, if you've a duplex scope, the subtension - distance between features of the duplex - can be used to range. The subtension will change based on magnification.
 
So let's say you shot at 8x zoom and made the follwing range card for your air rifle.

10m - 15 clicks down
20m - 10 clicks down
30m - zero
40m - 10 clicks up
50m -15 clicks up

And you did this with the objective/focus method.

Let's say you now bet with your friend that you can hit a penny at 50 yards.

Since a penny is a very small object, you want to see it very clearly. So you take your zoom up to the maximum 32x. Now you aim, with your clicks adjusted for the 50m, and you shoot...

will you hit?
 
1. If your scope does as I posted above about the scope on my Bushmaster.

2. If your rifle groups within a penny's diameter at 50 yards. (Or, if the first shot always goes into basically the same spot.)

After that it's up to you, as to reading the wind and controlling your sight picture and your trigger squeeze.

Art
 
"Trigger control" means that you play the game called "coordination".

If you just haul back on the trigger and jerk it, you ain't gonna hit nuthin'. You gotta be calm and gentle with it, just like with your favorite lady. And, you want to be just a little bit surprised when the gun does go off. (There's a line, there, but I'll omit it. :) )

Except on the most solid of benchrests with really good sand-bagging, there's always gonna be some wiggle of the gun. This means that your sights are always moving around, just some tiny bit.

Always remember that 0.2 seconds goes past between the time your brain says, "Fire!" and your finger actually pulls the trigger.

And this is where the coordination comes in. You must anticipate the sights covering the target, and tell your finger to pull, just before the sights are dead on. You will have already taken up any slack and put some amount of pressure on the trigger.

And that's trigger control.

:), Art
 
You are really overestimating the need to "click up and down" on the average scope.

Even with an air rifle your mid range trajectory should NOT require you to reset your scope at such short ranges.

For instance.. I hunt with a 30-06 with a 165 grain bullet. my "Zero" is at 200 yards. my "mid range" trajectory is 1.7 inches high at 100 yards (from my ballistics chart) now I COULD open up the scope, and give my self seven 1/4 inch clicks to re-zero BUT what they DON'T tell you is.. well sometimes its NOT 7 clicks. As you adjust the bullet impact the springs tighten and relax.. you need to "tap" the housing gently to make sure the springs aren't binding. Unless you buy a really high end scope, Unertl, Ziess this is a reality. Even with so called "traget knobs". You may also be interested to know MANY military sniper scopes are FIXED power, say 10x. To keep the shooter from trying to adjust too many things.

So follow my logic.. rather than clicking up or down you ZERO your rifle at say, 60 meters. Then, all things being equal you set targets at 20 and 40.. and just SEE where it hits. I doubt the difference will be as great as the heighth of a penny (if that) at such short range with a high powered air rifle. Now at 100 meters there may be reason to make adjustments (on an air rifle).

However, on a sniper rifle (or hunting rifle) there is VERY little need to adjust your scope up or down at close range.. say 100-300 yards. Why? Well with out even looking at my cheater card I cab tell you that at 25 yards the bullet is 1/4 inch low, at 50 its 1/4 inch high, at 100 its 1.7 inches high, zeroed at 200 and has a 7 inch drop at 300. Easy to memorize. If my range estimation is correct all i have to do to score a kill on say.. a deer is put the cross hairs on the kill zone. If I THINK I might be a little far off, I hold high ON the body, not over it. and at 300 meters/yards or less thats usually a 1 shot 1 kill situation. Well why is that? well a deer's body is say 15 inches high and the kill zone is about a 9-10 inch high triangle inside that. through practice I can more or less hold dead on at the top of that 'triangle" and even at 300 yards still punch the bullet through the kill zone.

So now, with an air rifle and a high powered scope, at 8x something only 20 meters away will look nice and sharp put the cross hairs on the penny PING at 40 yards hold at the bottom of the penny PING and at 60 hold dead center PING, at 80 you may have to hold at the TOP of the penny PING. No adjustments needed IF you know the trajectory of your pellet.

Plus there is trigger and breath control.

Start simple.. then move UP to complex.
 
So, if I understand you correctly Dr.Rob, you are saying that the trajectory of the pellet (air rifles) and bullet (real rifles) is straight enough up to 60 yards and 300 yards respectively.

So you are saying memorize the little adjustment you must make, such as hold a little high or a little low, and then you won't have to ever change the scope settings?
 
Pretty much, SS.

What The Dr's refering to is called maximum point blank range. The distance at which you can hit a vital zone without making any adjustments - either of the scope's internal settings or where you place the cross hairs on the target. A bullet has a curved path (up & down) & within a certain range & aiming dead center of the target, that bullet will stike some place vertically on the target. For a "standard" .30-06 hunting round, you can expect maximum point blank range to be right around 270 yards (or so) for a 6" target. The bullet never "rises or falls" above or below the center of the target - a +/- 3"

Sounds a bit confusing & I'd suggest taking a look at any on-line (& free ware) ballistics software so you can see a picture. Easily worth 1K words in this instance.

For your airgun - no telling what ballistic path you're getting. Only way to tell would be to set up your penny-sized targets at the desired ranges, shoot 5 times (or so) at the center at each range & record how high or low the BBs/pellets hit at those ranges.

Set the zero of your scope to a "kinda mid-point" range & any adjustments thereafter, make with setting where you place your cross hairs - either a bit high or a bit low.

I agree that it sounds like you are making this a lot harder than it needs to be. & no fault in that - sounds like you're asking good questions, too.

Does your scope have a duplex cross hairs? Duplex is the centers are fairly fine, while the outer portion of the crosshairs are thicker. If you do, I'll try to explain that subtension thing for ranging, if not, I'll just go away .... ;)
 
Don't have scope or airgun yet. I'm stil searching...

As for the scope, I was thinking a Swift 8-32x50.

I think it does have rangefinding ability and is pretty good for the double-recoil...

still debating on on Swift/Bushnell/Pro Air scopes...

I'm one undecided person aren't I :D
 
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