Question about Hammers Air Gun Rifle Scope 3-7x20 With Rings

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How to use the BDC on my rifle scope

Can you tell me how the BDC works on the 3-9x42gdt Hammers Rifle scope. I know it is for the 223 but not sure what grain bullet.

Thank you in advance

Leon

Posted by twinbard on

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  1. Step 1 Take a rifle with a properly installed BDC bullet drop compensating reticle to a rifle range. Using ear protection zero the scope reticle so that the point of impact corresponds to the center of the cross hairs at the distance you have selected - typically 100 or 200 yds.
  2. Step 2 Determine the trajectory of the specific cartridge you have selected. There are several ways to do this. Ammo manufacturers publish trajectory and wind drift information. There are web based ballistic calculators like http://www.biggameinfo.com/BalCalc.aspx which will tell you how much your bullet drops at known distances.
  3. Step 3 Scope manufacturers like Leupold can be a valuable source of information about their reticle: "Leupold® Ballistic Aiming System: Boone and Crockett Club® Big Game Reticle aiming system provides a series of additional aiming points to improve your ability to shoot accurately at longer ranges. NikonHow to use the BDC on my rifle scope - 2_bing.gif also provides good information suggesting the marks on their reticle be used for zero at 100yds followed by circles below representing 200, 300, 400 and 500 yds if the cartridge travels around 2800 ft per sec. Nikon suggests the center cross hair be zeroed at 200 yds for magnum calibers traveling around 3000 ft per sec. We understant that each variation of different bullet weight and powder charge changes trajectory and a scope manufacturer can not build a different reticle for each different cartridge made so practice on the range to determine how well the marks relate to the actual impact of where your bullet strikes at a know distance is important. The one thing that people using BDC scopes typically have problems with is that a BDC scope has the reticle in the second focal plane of the scope. If the reticle was in the first focal plane of the scope the reticle would look smaller on low powers like 3x and grow proportionately larger as the power increased to say 9x top power. The problem is that while the marks on the BDC reticle correspond accurately to the bullet drop at the know distances 200, 300yds etc. What happens when you lower the power from the scopes maximum power to any other lower power is the reticle stays the same size and the field of view within the scope increases which means that the distance between these marks on the BDC reticle no longer corresponds to the point where the bullet will strike. In short BDC reticles only work at the maximum power of the scope or at a set specific power. At all other powers these BDC reticles do not accurately represent where the bullet will strike.
  4. Step 4 The center X always remains the same. If you zero at 100 yards and you know that your bullet drops 8 inches at 300 yards you could forget about the BDC marks and hold the center X 8 inches high - that works at any power 3x or 9x and should be used at lower powers. If you zero the center crosshair at 100yds and have the BDC scope at the maximum power 9X then the first line or circle below the center X should be the mark you place on the center of the 200 yard target----- the bullet strike should hit the center. If by some chance you put the scope on 3x and placed that first mark below the center cross hair on that 200 yard target you would shoot over the top of the target. This is because as the power of the scope decreases the field of view increases the angle increase and gets wider. You can experiment with known power settings and see at a specific power say 3x what that first circle down corresponds to and make notes because at any set power what the marks correspond to will be repeatable.
I got this from a website, hope it helps.

Posted on May 04, 2010

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