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Leupold’s Ballistic Aiming System (B.A.S.

Printed From: OpticsTalk by SWFA, Inc.
Category: Scopes
Forum Name: Rifle Scopes
Forum Description: Centerfire long gun scopes
Printed Date: July/16/2018 at 05:46
Software Version: Web Wiz Forums 11.10 -

Topic: Leupold’s Ballistic Aiming System (B.A.S.
Posted By: Chris Farris
Subject: Leupold’s Ballistic Aiming System (B.A.S.
Date Posted: May/04/2004 at 12:58

Here is a Flash animated instruction / tutorial on how the two new bullet drop reticles (Boone & Crockett and Varmint Hunters) work. - Leupold B.A.S.

SWFA, Inc. - - - -

Posted By: Chris Farris
Date Posted: September/08/2004 at 09:09

Range estimation of various game animals with the BAS Reticles


The technical service department has been receiving a number of inquiries regarding the possibility of using the range estimation feature on the power selector ring to estimate the range of animals other than deer.  This inquiry has most commonly come from those with a Varmint Hunter’s reticle who would like to estimate the range of a coyote, or those with a Boone & Crockett reticle who would like to estimate the range of an elk.  A duplex reticle makes it impossible to range animals other than deer, but the addition of multiple reference lines in the BAS reticles dramatically increases this potential.  I have done some preliminary testing and developed a method of using the power-selector range estimation feature to range coyotes, antelope, deer, elk, etc. using either of the BAS reticles. 

To obtain the information below, I followed a rather simple method using a 100-yard distance (in many cases 100 yards is the only measured distance people can find).  Since the low magnification setting on the scope corresponds with the 200-yard mark for range estimation, I turned the scope to low magnification and viewed a ½ -size target at 100 yards.  Since 100 yards is half of 200 yards, I viewed a black square ½ the size of the animal I want to range (a 26” elk chest at 200 yards will appear 13” at 100 yards), so for elk I used a 13” black square at 100 yards.  I then viewed the square and made note of which two lines vertically bracketed the square exactly – in this case the Boone & Crockett reticle lines for 300 yards and 450 yards and the Varmint Hunter reticle lines for 200 yards and 400 yards aligned perfectly.  This means that to range an elk with the Boone & Crockett reticle you can simply adjust the magnification until the 300 yard mark is aligned with the top of an elk’s shoulder and the 450 yard mark aligns with the elk’s brisket and read the range indicated on the power selector.


For other animals I used the following size criteria:

Coyote from feet to ear tops (alert position) – 24”

Buck Antelope chest – 16”

Large Buck Mule Deer chest – 24”

Average Elk Chest – 26”


To range the animals listed above using the Boone & Crockett reticle use the following marks:

Coyote – 200 yard mark and 300 yard mark

Antelope – 300 yard mark and 400 yard mark

Mule Deer – 300 yard mark and the top of the 450 yard mark (top edge of the line)

Elk – 300 yard mark and the bottom of the 450 yard mark (bottom edge of the line)


Using the Varmint Hunter’s reticle:

Coyote – 500 yard mark and the range estimation mark (not the top of the duplex post)

Antelope – 300 yard mark and the 400 yard mark

Mule Deer – 200 yard mark and the top of the 400 yard mark (top edge of the line)

Elk – 200 yard mark and the bottom of the 400 yard mark (bottom edge of the line)


Many people will not agree with the size measurements I have used, so they can simply estimate the size of the target and follow the same method I used.  For example if they feel a more accurate measurement for a mule deer chest is 20”, they can simply place a target at 100 yards which measures 10” (rather than 20” because you are simulating that the target is placed at 200 yards) and see which lines bracket the target best when the scope is set to low magnification.  Those two lines will then be used to bracket the animal, making the range indicated on the power selector ring accurate.


- Leupold Technical Service Staff

SWFA, Inc. - - - -

Posted By: sscoyote
Date Posted: October/05/2004 at 21:33

Chris-- several yrs. back i got acquainted with a guy that recalibrated the Leupold Range Estimating System in their scopes for a coyote-sized target. The system he came up with was fascinating so i went home with my mind going 200 mph. The whole system of 2nd focal plane reticle subtension vs. magnification is based upon a simple linear ratio and proportion formula, i.e. as magnification changes so does reticle subtension, but as magnification increases reticle subtension decreases, so the formula is actually inversely proportional. This stuff was really neat so one day while i was sitting at my dining room table i looked out the window at the road that runs straight away from it to about 1000 yds. or so with various size signs along it at various ranges. I though why not check the system on these signs. So the next day i went out with a tape and my laser to (discreetly) measure the signs and their distance from my window. I then looked up the various reticle subtensions on the scope makers website under the technical page. Then i calculated what magnification i would need for each sign top to bottom and left to right. I could not believ it but it was dead on most of the time. When they were off some i just changed the magnification till they were bracketed, and used that as a "fudge factor", and then they were always right on. Now some oculars aren't calibrated well so I would have to interpolate as best i can-- but it still seemed to work pretty well. But there was just 1 problem-- some of the scopes didn't have any 100 yd. subtension info. on their website, so i couldn't do anything with them precisely. Awhile later i was riding my bike down the road when i happened to stop at a stop sign, and found the answer. Most street sign posts have holes drilled in them @ exactly 1" intervals all along the post. As i rode back to the house i noticed a street sign that appeared to be about a hundred yds. from my dining rm. window-- laser said 102. PERFECT! Now all a buddy of mine needed to do was bring his scope over, i could measure reticle subtension on that sign post from my window, and calibrate the reticle for ranging and ALSO multiple downrange zero reference by matching up each reticle subtension change with the correct magnification change for each 50 yd. interval out to as far as the variable power scope would allow me to go (as long as the shooter knew his mv, and sight height above bore, i could calculate the rest).


For field use tho, i don't like Leupolds RES. They suggest to change the magnification to range a deer to 600 yds. as i remember. I think a better way to do it is to establish the CONSERVATIVE maximum point blank range (MPBR) for whatever species u're after, and then adjust the scope to whatever power will work as a ranging tool to that range only, and then interpolate accordingly. here's what i did with a buddy's mil-dot system just to day. He's going after cow elk with a 300 WM, 180 gr, SGK @ 3075 fps. I calculated his CONSERVATIVE MPBR @ 350 yds. Now the reticle subtension between mildots is 3.6 MOA. Turns out 2 mil-dots then is 25" @ 350 yds---- 7.2"X3.5=25" Now all he has to do is bracket the elk between 2 dots, and if it's as big or bigger than the bracketing gap just aim dead center and shoot-- dead elk (hopefully). If the elk is bracketed at 1 mil, then he's probably too far away to shoot @ 700 yds. 1 1/2 mils must be 350 plus 1/2 350 or 525.


This stuff is really fun to calculate on your own if u're into math at all. It sort of establishes a "tactical" type system for field use.


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