30/30/duplex reticles and rangefinding

[fat tony]

How do you use one of these for rangefinding? I understand that on some, the fine crosshairs represents 30 inches at "X" magnification at "X" distance(often 100 yards, I would guess that the pamphelet the manufacturer gives you gives directions on how to estimate holdover as long as you know the range? Otherwise, is there a site that will give you a good more or less education on how to calculate the holdover, within reason?

[sscoyote]

Tony, the plex (30-30) reticle is the simplest of all ballistic and rangefinding reticles, IMO. I apply the plex reticles for rangefinding and downrange zeroing and windage on any scope i happen to be using. Here's how it works--

Let's take a look at some of the Leupold Duplex reticles as an example. If u've ever noticed many of the Leupold scopes have a rangefinding tool built right into the power ring. The system is designed such that the avg. buck deer is supposed to be close to 16" back to brisket. What u are supposed to do is when a buck gives u a broadside profile than adjust the power ring until he fits between the x-hair and plex post tip, then read off the yardage on the opposite side of the power ring. This what they refer to as their range estimating system (RES). The system does have some feasability, but there is a better system that allows the user to apply his plex reticle with many different size tgts, and u don't have to adjust the power ring to make it work. It'll work with any plex reticle, and really any ballistic reticle as well (Ballisitc Plex, Nikon BDC, whatever u're using). It's also faster to apply.
The system is based on the mil-dot mil-ranging formula, but just adapted for the plex measurement (subtension). Here's the formula--

Tgt. size (inches) X range of reticle subtension measurement (usually 100 yds.) / reticle subtension (inches) / quantity of gap tgt. brackets (tenths of total gap) = range (yds.)

Now this formula looks complicated, but in practice it's quite simple.
Ok let's look again at the Leupold Duplex. If u have the Leupold catalog it gives u the measurements (subtensions) of all the plex reticles in the Tech. section. at both low and high power. Suppose u have the 3.5-10X VX-III for instance. That plex reticle is 1 of the RES systems noted above. The subtension of the Duplex is 5.4 inch per hundred yds. @ 10X plex post tip to plex post tip. Let's see how it works when we plug all the #'s into the "modified mil-ranging formula" above--

16 x 100 / 5.4 / 1.0 (deer brackets right at 1 "plex unit") = 296  yds.

Now let's finish the ranging chart for our 16" deer recognizing that 16x100/5.4 is a constant of 296, and we can enter that into our calculator's memory--

296 / 0.9 (deer brackets 90% of our plex unit) = 330

296/.8=370
296/.7=423
296/.6=493
296/.5 (x-hair)=592

Now, u may notice a couple things here. 1st off the .5 mark is the x-hair which is exactly the system that Leupold uses for their RES system above. Another thing that can be seen is that the difference between guessing 6 tenths of a plex unit and 1/2 a plex unit is a whopping 100yds. This should tell u that as range incresases so does the inaccuracy of the system. It's an angular system of measurement and it's not perfect. Gotta go-- wife's home. If u go out and measure your 30-30 reticle at 100 yds. (or even 50 yds. will work--but try to get as accurate to .1 inch as possible), u can try it yourself with gallon milk jugs set out at distance or something else, whatever. It's a kick to play with , and will blow your mind how well it works on tgts. whose dimensions don't change.

[sscoyote]

Here's how the plex can be applied as a ballistic reticle for both drop and windage--

1 of my rigs that i hunt coyotes with is an XP-100 .17 Mach IV pistol, that has an effective range of about 300 yds. on coyotes. I have the Burris 4-12X Compact fine plex reticle on top. The plex subtends (measures again) 2.85 inch per hundred yds. (from the Burris website or catalog). This equates to 2.7 minute of angle (MOA) since there are 1.05 inch per hundred yds. to each MOA. I convert the subtension since most ballistics programs calculate drop and windage in MOA. Now i simply run a ballistics program and calculate drop in MOA. Turns out that with my load, and sight-in distance of 225 yds. at 340 yds. the bullet drops 2.7 MOA That is now the zero of the plex post tip. Now go out and confirm it at the range. If it jives then go back to the ballistics program and calculate the rest of the drop chart in 25 yd. increments, like this as an example--

300 yds. = 1.62 MOA drop. If we divide that by the total subtension of 2.7 that equals .6 Now if a coyote is lasered at 300 yds then aim 6 tenths of the way down to the plex post tip, and kill him.

Now do the same for windage which is typically calculated as 10 mph drift from 3 or 9 o'clock. Since the .17 caliber is particularly wind sensitive (lower BC bullets) at 300 yds. for my load i would have to aim 1.4 "plex units" into the wind.

Now here's what that entry would look like on my range sticker that's attached inside the ocular Butler Creek scope cap cover (and laser)--

300-.6-1.4

No headers need to go on the sticker as it's obviously--
1) Range
2) Vertical
3) Horizontal

This is an important issue as the range sticker should be as simple as possible.

2 years ago i was presented with just such a 300 yd.  opportunity in a 10 mph 3 o'clock wind, and was able to kill the dog with my system. It does work, quite well in fact.

Many feel they need a ballistic/ranging reticle for longer range shooting to intermediate ranges, when they may have the perfect reticle already right in front of their eyes that very few every realize the full potential of. I didn't either until i started investigating the possibilities.

[fat tony]

SSCoyote, thanks for the education! I love this forum! Regards FT.