Percision plex Recticle

[corbin9191]

I just got a new pentax scope and I got a chart with the guns and the loads for this scope so it will work with the recticle. The only problem is that for my 300 win mag they want my bullet to go at 3290 fps with a 150 gr bullet, but the chart says the altitude is "oft" and where I am the alititude is about 4500 feet and all of the ballistics charts that i am using says my bullet would have to go at 3000 fps to match what the bullet is droping for the recticle to work. The question that I am wondering is will this recticle still work if I am going by what the paper say's? Thanks

 

[old #7]

Call me stupid but what does 'OFT' mean.
I think that you will have no problems matching the ballistics required for your reticle to be accurate.
Do you have a chronograph or access to one and also a ballistic program?
I live @ 90ft above sea level and target shoot at 200ft.
If I were to shoot these same loads at 4500 ft they would not slow down as fast and consequently not drop as fast because there is less "air" at 4500ft than 200 ft.
So... If anything you might have to lighten up your loads at your elevation to match the reticle.

 

[Guest]

oft is very likely zero feet.

Now, as to whether or not the reticle will work, I can say this much: Regardless of what is written on a piece of paper regarding which loads to use and so forth, because every rifle is different, you'll need to shoot the rifle at each range to determine how close it is to dead-on. Otherwise, you're just guessing what your rifle is doing (in terms of velocity) and even if you chrony'd it, you're still guessing what the bullet is doing in terms of real world external ballistics (trajectory), which is entirely what I believe you're trying to avoid in having the multiple holdover points. So go to the range, zero it in, and then shoot it at the various ranges and find out. That's the only way to really know. Ballistic reticles are great ideas, but much like boresighting, they are tragically misunderstood in terms of the use and many hunters believe they are just what is written on the paper, and that shooting at those distances will now be easy. It may be easier, but you'd better verify it. You could find your load off as much as 6" or more at 300+ yards, regardless of your altitude.

Sorry if this sounds like a rant, I'm a little tired. Forgive the harsh tone, but understand the idea is to verify your gun's performance with this reticle to know what it will do at various ranges.

 

[old #7]

dubyam wrote

oft is very likely zero feet

I bet your right.
It should read, 0 ft, for those of us that didn't stay at a Holiday Inn Express.

 

[Antelope_Sniper]

Dubya is right about needing to experiment with those fancy reticles at the range. Personally I use mildot reticles. Because they are a know standard, I can use them with any load.

Here's my suggestion.

Get your self a gradiated target, such as the Redfield sight in targets. I like them because it's exactly 1" between lines on the target.

Set it up at 100 yards, and measure the distance between your main cross hair and each hold over point in inches, at 100 yards.

This will give you a good aproximation of how many MOA each hold over point represents.

Chronograph your load, and plug your data, including altitude, it into a good balistics program such as this one:

http://www.eskimo.com/~jbm/calculations/traj/traj.html

Figure out at what range your hold over points should be on, and go find yourself a nice long range to test it out. Like Dubyam said, until you test it, it's all just a big guess.

 

[old #7]

I agree that "real world" situations are far different than the scientific calculations that were made in developing the ballistic retical.
It would be pretty tough to exactly match the conditions that were used in the design of the retical out in the field.
But lets face it most hunters don't take a altimeter, humidistat and barrometer in the field while hunting.
More than likely the manufacturer designed the retical around the ballistics of a few common rounds.
The more you try to macth the ballistics that were used in the development of the retical the better the retical is going to perform.
An example would be........
I have a retical that was designed arount the .284, 140gr Nosler AccuBond with a MV of 3150fps. The retical was desinged for a 200yd zero and it has dots representing 300, 400 and 500yds.
If I use a different bullet, lets say a 175gr Partition with a MV of 2800, the retical won't be close.


Your best bet is to macth your load to the load used in the design of your retical.
I definately agree that you should test it at th range before counting on it in the field.

 

[Antelope_Sniper]

True, but if you use my method, you can figure out at what ranges it is accurate.

 

[Antelope_Sniper]

Here's the basis translation:

Bullet 150gr NBT.
muzzle velocity of 3000 fps
altitude: sea level

If initial zero is 100 years and hash marks are at
200
300
400
500 yards

same bullet
muzzle velocity of 3290
altitude of 4500

Hash marks are now on at:
225
350
467
590
Your results may vary.

 

[old #7]

Why not load the bullet to 3000fps @ 4500ft?
Thats the beuaty of hand loaded ammo that is tailor made for specefic situations.

 

[Antelope_Sniper]

Good question 7, Well, for a shooter like me, the extra 300 fps expands my usable range from about 525yds to 625 yds. Out in the middle of the Wyoming prairie, that extra 100 yds might come in handy.

 

[corbin9191]

Dubya is right about needing to experiment with those fancy reticles at the range. Personally I use mildot reticles. Because they are a know standard, I can use them with any load.

Here's my suggestion.

Get your self a gradiated target, such as the Redfield sight in targets. I like them because it's exactly 1" between lines on the target.

Set it up at 100 yards, and measure the distance between your main cross hair and each hold over point in inches, at 100 yards.

This will give you a good aproximation of how many MOA each hold over point represents.

Chronograph your load, and plug your data, including altitude, it into a good balistics program such as this one:

http://www.eskimo.com/~jbm/calculations/traj/traj.html

Figure out at what range your hold over points should be on, and go find yourself a nice long range to test it out. Like Dubyam said, until you test it, it's all just a big guess.

Would I have to use that technique at only 100 yards or could I do that at 200 yards because I will be sighting in my rifle in at 200 yards?

 

[Antelope_Sniper]

You certainly could. And in reality, with your scope, that's probably what I would do.
I just didn't want to make my example too confusing.

 

[old #7]

I believe that you will need to have your scope magnificatian all the way up to use Antelope Sniper's measurement technique.
The Mill dot retical scope I had for a while measured 3.6 inches between dots @ 100yds when it was at full power? There was also an easy way to use the retical as a range finder but, I don't remember the details.

 

[Antelope_Sniper]

Yes Old 7, most mildot/plex reticles are calibrated on the highest power, however, some Mildot scope's are calibrated at 10 power. There are a few "first plane" reticles out there, where they are alway calibrated regardless of the magnification, but they are the exception, rather then the rule.

 

[corbin9191]

I believe that you will need to have your scope magnificatian all the way up to use Antelope Sniper's measurement technique.
The Mill dot retical scope I had for a while measured 3.6 inches between dots @ 100yds when it was at full power? There was also an easy way to use the retical as a range finder but, I don't remember the details.

To use the scope as a range finder when you have to mildot recticle you need to know the height or width of your target then multiply that by 27.778 and take that answer and divide it by how many mils the target measures.

 

[Antelope_Sniper]

Yea, mildots can be a bit of an art. Since I don't take a calculator with me when I go to the field, I always try to measure on something that is about 3 feet.
Since 36" is one mil at 1000, I just have to divide 1000 by the number of mils I measure.
1 Mil = 1000 yds
1.25 = 800 yds
1.5 = 666 yds
2 = 500 yds
3 = 333

Since I zero my rifle at 100 yds, but carry it with +2 1/2 MOA on the dial, 3 Mils or more just mean hold on the critter and pull the trigger.