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Old January 8, 2019, 09:25 AM   #26
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I've done it. The whole exercise is a little fuzzy since you have to decide where the center of the groups is, and you may also have other things coming into play like subtle changes in your shooting position.

At the end of the day, it's about as accurate as making an educated guess on MV based on available loading data, etc.

It's an interesting exercise. And it brings home the concept that what we view as hard numbers are in fact a lot more flexible than we would otherwise think. everything has its variations, and nothing is truly a fixed number.
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Old January 25, 2019, 04:25 PM   #27
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I have spent the time to get a more accurate measurement of scope height. Then using Actual Bullet trajectory at 100, 200 and 300 yards. Along with Bullet specs. 3100 FPS is the value that basically fits the bill to within 1/4 inch at 300 yards on the trajectory. I am gonna set my guess for bullet velocity at 3100. Then get actual readings with my new chronograph in a couple weeks.

So here are the deats.....

270 wsm, 22 inch barrel
Nosler 140 grain BT .277 Caliber. G1 BC .456
67.5 Grains of Magpro
cci 250 LRM primer.

Scope height= 1.785 (Rounded to 1.8" for the calculator"
Zero= 200 yards
Zero height= 2.5"
Height at 100 yards= 2.6
height at 300 yards= -2.5

I used the actual Atmospherics for my location at the time the groups were shot.
21 degrees F
Elevation 800 Feet
pressure 30.09"
Humidity 43%

The Ballistics Calculator From JBM Ballistics follows that trajectory with less than .20 inches deviation Throughout my target ranges. When plugging in a velocity of 3100 FPS... I can not wait to get a new chronograph to compare it to. That velocity is within the normal range for the bullet weight and Cartridge. Also about 90 FPS Slower than the manual has the velocity with that powder charge.

Last edited by kilotanker22; January 25, 2019 at 04:30 PM. Reason: Spelling
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Old February 10, 2019, 10:23 PM   #28
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Originally Posted by kilotanker22 View Post
I have been pondering lately.

If we use Bullet data and muzzle velocity to calculate trajectory. The We could remove one known variable from that. Replace it with another and still solve for the missing variable. (pretty basic concept)

I have not had the motivation to go that route until now. (I Like to do things the lazy way..) Alas My Chronograph is broken, I have a new rifle that I have not yet tested. I am also saving up for a dopler radar setup.

I am gonna zero the rifle with factory ammunition. Use the factory ammunition data to calculate a zero. For the Ammunition I am using. Bullet velocity is 3100FPS. G1 BC is .61. The factory data would work out for me having a zero of 24 yards, This would also zero me at 300 yards. (for elevation that is)

Plug in the factory data for MOA adjustment at ranges from 24-500 yards.

Then shooting several groups at each of those ranges. Then using the Test data to compare to Factory. Using your test data (Actual trajectory) and bullet data. I intend to Calculate my actual muzzle velocity. Then re zero and confirm.

Then When I get That Radar. I can test my calculations against it. I know that this is totally doable. I am gonna do it, mostly because I am bored. And to start a conversation here. (When I nothing else to do.)
The problem w/ this is the BC.

It represents the variable "half-life" of muzzle energy, based on any given velocity, which is also a variable.

Some manufactures use a 100 yard velocity, some use a 300 yard velocity - giving an envelope fairly accurate to twice the chosen range. But as velocity changes, so does the BC.

While interesting, this endeavor sounds, ballistically, like oral proctology.

Get a Chrony.

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Old February 11, 2019, 01:27 AM   #29
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I bought a magnetospeed.

My estimated velocity was about 75 fps faster than actual.
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Old February 11, 2019, 07:19 AM   #30
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While you have a chrono now, don't take it as gospel.
75 fps is within the realm of error for a chrono.
You need to verify with trajectory at longer distances.
If your shooting at 100 yards, stretch out to 600 and see if the ballistics works out.
Strelok has truing options in their calculations. Mostly i choose velocity, but there is an option to change the BC for truing.
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Old February 11, 2019, 11:01 AM   #31
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Yeah, I think that While I am saving up for A radar setup. I will just have to break down and buy a prochrono or something for the time being.
Ok, the Doppler is what $500 or so? Why waste $110 for a Chrono and then pay more for another one?

Get a loan darn it!!!!!

I do like the thinking and testing idea. Appreciation for how hard it used to be.

A Chrono was not even a dream back when I started serious shooting.
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