Saturday, June 26, 2010

Leupold RX-1000 TBR Rangefinder Review - Part 4: True Ballistic Range (TBR) Rifle

Leupold Review, RX-1000 Review, RX-1000 TBR Review, Range Finder Review, Leupold Rangefinder Review
In this part of the review, I'm going to cover some of the different True Ballistic Range (TBR) Rifle features of the Leupold RX-1000 TBR Rangefinder.  These are some of the items covered.
  • True Ballistic Range - Hold
  • True Ballistic Range - MOA
  • True Ballistic Range - BAS
As I discuss these features, I will use the "AC" group which includes the .308 Winchester 168 grain bullet at 2670 ft/sec.  I selected this groups because it matches one of my rifles (FNAR).

The first thing I wanted to do was understand the basic bullet drop equation within the rangefinder.  To do this, I used the "Hold" mode so that it would display hold values.  The hold values are the inches above or below the target that I would hold and are basically bullet drop values.  I found a flat area and measured these values for every inch of hold between 200 and 500 yards.  I also included some data on out to 700 yards for approximately every 5 inches of hold.  You can see this data in the graph below (clicking on the graph will bring up a larger image).  I also plotted the ballistics data for the .308 Winchester 168gr Ballistic Silvertip (2670 ft/sec) which is shown by the red diamonds.  This data lined up very good with the Hold data from the rangefinder.  Since they also stated in the Leupold Rangefinder Owner's Manual that this group also contains the .308 Winchester 150gr (2820 ft/sec), I plotted the ballistic data for the Winchester Power-Point shown by the green diamonds.  Notice that there is a significant difference.  I have to assume that Leupold must be referring to a bullet with a different ballistic coefficient than the Power-Point due to the mismatch.  I had MS Excel create a trend line and I have shown the equation for this line on the graph.  If you are using Group AC, I recommend plotting this line and then comparing the rangefinder data against the actual ballistics for your cartridge.

The next mode I checked was the "MOA" mode.  Again, I found a flat place and checked this mode between 200 and 500 yards for every MOA value which ended up being 1 through 9.  Since the rangefinder does not show any fractions of a MOA, I also made sure I found where the yardage change was between each MOA value.  You can see this data in the graph below shown in blue.  This data looks like a stair step because a single MOA value covers a range of yards from 39 yards at 1 MOA to 25 yards at 9 MOA.  Because of this range per MOA, there will be error introduced.  The error introduced will be a function of the ballistics of your cartridge versus where the tarted actually falls within the range.  I included in the graph below the ballistic data for the .308 Winchester 168gr Ballistic Silvertip (2670 ft/sec).  Just like in the "Hold" data, you can see this appears to be a good match.  Again, I included the .308 Winchester 150gr Power-Point (2820 ft/sec) to show that you need to be careful to check your cartridge and make sure it falls within the ballistic range recommended by Leupold.

To better understand what this range means from a drop perspective, the data below shows the bullet drop range for each MOA.  For example, if the rangefinder says 4 MOA and you adjust the scope 4 MOA, the bullet drop range is 4.9".  From the graph above, the 168gr bullet falls near the mid range of the 4 MOA range.  This means that the total range is 4.9", so the variation in my bullet point of impact may be +/- 2.45".  Out at the 9 MOA range, it would be +/-3.65.
  • 1 MOA, 220-258 yards, 2.8" bullet drop
  • 2 MOA, 259-295 yards, 3.6" bullet drop
  • 3 MOA, 296-329 yards, 4.2" bullet drop
  • 4 MOA, 330-362 yards, 4.9" bullet drop
  • 5 MOA, 363-393 yards, 5.4" bullet drop
  • 6 MOA, 394-423 yards, 6.0" bullet drop
  • 7 MOA, 424-451 yards, 6.3" bullet drop
  • 8 MOA, 452-478 yards, 6.8" bullet drop
  • 9 MOA, 479-504 yards, 7.3" bullet drop
Personally, I wish Leupold would have come up with some way to show fractions of a MOA to match my 1/4" adjustment capability of my scope.  This 1 MOA increment may be OK for big game hunting, but I think it could be better.

The last rifle mode is the "BAS" mode which is to be used with Leupold's Ballistics Aiming System® reticles or gives the equivalent horizontal range.  In this mode, if there is no up or down angle for the shot, the TBR range will match the LOS (line of sight) range.  The rangefinder is taking the LOS range and the angle to the target to determine the range you should use for shooting.  Since the rangefinder measures and uses the angle in the calculation, I needed to determine the accuracy on the angle measurement.  I set the rangefinder up on a tripod and measured the horizontal (0 degree) distance to a wall and marked that location on the wall.  Next I rotated the rangefinder down in 2 degree increments from 0 to 54 degrees down and marked each location.  Last I measured the distance on the wall from the 0 degree mark to each 2 degree increment.  After doing the math on this data, it worked out that every measurement was within 1 degree, therefore I believe the angle measurements to be within +/- 1 degree.  Since mine was not an overly precise method, it may be even more accurate, but this is good enough for this type of a device.  Since I was already setup, I checked the up angle accuracy to 20 degrees and again the accuracy was the same, +/- 1 degree.

Now that I have established that the LOS accuracy is good and the angle measurement is good, I have to believe that the equivalent horizontal range (TBR in the BAS mode) will be good since this should only be a simple calculation if you use the Rifleman's Rule found at Wikipedia.  The TBR range should be the LOS times the cosine of the angle which give the horizontal range.  Since I don't live in a mountainous area, for now I'm going to take Leupold's word on this mode.  If I get any data that says this is not the case, I will update this part of the review.

You can see other parts of this review by going to the Leupold RX-1000 TBR Rangefinder Review Summary Page.

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