Pennsylvania Firearm Owners Association
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  1. #1
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    Default Bizarre Scope Behavior

    This is driving me crazy!! I am hoping someone can help me understand a bit of crazy optics behavior. I have a Nikon Buckmaster 6-18 side focus on a CZ 452. The minimum parallax adjustment is 50 yards. I am shooting in a small area, so I am using Remington CBee ammo (33gr, 740fps). Currently, I have zero at 20 yards (I don't normally shoot in such cramped conditions). The mystery is that if I use the same setup, I hit one inch low at 10 yards. According to everything I have referenced, I should be about .4 inches low at 10 yards. How is it possible to be on zero at 20, yet mysteriously too low at 10? Is this some artifact of shooting too close for this scope? Any guesses???

  2. #2
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    Default Re: Bizarre Scope Behavior

    Quote Originally Posted by Redvole View Post
    This is driving me crazy!! I am hoping someone can help me understand a bit of crazy optics behavior. I have a Nikon Buckmaster 6-18 side focus on a CZ 452. The minimum parallax adjustment is 50 yards. I am shooting in a small area, so I am using Remington CBee ammo (33gr, 740fps). Currently, I have zero at 20 yards (I don't normally shoot in such cramped conditions). The mystery is that if I use the same setup, I hit one inch low at 10 yards. According to everything I have referenced, I should be about .4 inches low at 10 yards. How is it possible to be on zero at 20, yet mysteriously too low at 10? Is this some artifact of shooting too close for this scope? Any guesses???
    The answer to your question is that the bullet is still traveling upwards and has not reach the peak of the trajectory at 20 yards. Since the bullet is traveling upwards, at 10 yards it has not traveled "up" as much as it has at 20 yards. Remember that the trajectory of the cartridge isn't a laserbeam straight line, it's a curve. The place where you're zeroed just means that the sights line up perfectly with the curve at that distance.

    The other thing to consider is that the scope is usually not perfectly parallel to the bore line, there's an angle between them as well. Depending on this angle, which is also related to bases, action, barrel, etc. this will change and effect the point of impact. I think it's interesting that you think it's a set value to be so many inches low or high at a particular yardage. Have you ever heard of "sight height"? Let's say you had a piece of paper with a dot on it at the end of your muzzle, and not any further. If you put the crosshairs perfectly on the dot and pulled the trigger, how far would it be? Probably an inch and a half low, maybe more, or maybe less, depending on your rig. This will vary depending on the scope, objective size, ring height, bases, etc. So what you're seeing is the bullet climbing up towards the top of the trajectory as it goes through different yardages. It's nothing that you should worry about, but just something that you should document if you also want to shoot 10 yards. I personally don't quite see the benefit of shooting at 10 or 20 yards with a scoped rifle. I guess if you want to shoot some rounds and have no other option, then that would be an explanation. If you're trying to get some practice in, I'd say that using a NON rimfire rifle, and doing some dry firing my benefit you more. You should NOT dryfire with your CZ452 because it's possible to damage the chamber because of too much firing pin protrusion. At 10-20 yards, you just have to move the rifle a lot to throw a shit anywhere. Just some things to think about anyway.

    In short, the difference your seeing is related to your particular rig. You're seeing the difference in the sight height (scope mounted height vs. bore height), and the trajectory of the bullet going upwards. Hope this helps.

  3. #3
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    Default Re: Bizarre Scope Behavior

    That's what I screwed up - the sight height! I never changed the setting in my ballistics software, so it was yielding a calculation based on 1.5 inches, rather than 1.9 inches. Boy, do I feel stupid. The reason I am firing a scoped rifle at those ridiculously low ranges is nothing more than boredom combined with the need to not make much noise. I could pull the scope off this rifle, but the CZ 452 Varmint does not have iron sights.

  4. #4
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    Default Re: Bizarre Scope Behavior

    Quote Originally Posted by Redvole View Post
    That's what I screwed up - the sight height! I never changed the setting in my ballistics software, so it was yielding a calculation based on 1.5 inches, rather than 1.9 inches. Boy, do I feel stupid. The reason I am firing a scoped rifle at those ridiculously low ranges is nothing more than boredom combined with the need to not make much noise. I could pull the scope off this rifle, but the CZ 452 Varmint does not have iron sights.
    You shouldn't feel stupid, it's something that lots of people overlook. I'm guessing that it's giving a number right about where you'd expect now? No problem with firing a scope rifle at shorter ranges, especially if it's just because you're bored and want to. If you like shooting it, go for it. It's a beautiful rifle, and I'd like to get one myself sometime. Glad that helped.

  5. #5
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    Brookville, Pennsylvania
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    Default Re: Bizarre Scope Behavior

    Like Tomcat described.. Its sometimes easy to forget about the scope being higher than the bore, and the bullet has to be aimed up to meet the straight line-of-sight that the scope has.

    See my crappy MS Paint drawing.
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    RIP: SFN, 1861, twoeggsup, Lambo, jamesjo, JayBell, 32 Magnum, Pro2A, mrwildroot, dregan, Frenchy, Fragger, ungawa, Mtn Jack, Grapeshot.

  6. #6
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    Default Re: Bizarre Scope Behavior

    well stated, tomcat, i was just thinking the rise in trajectory but hadn't thought much about scope height.

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