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January 8, 2019, 09:25 AM | #26 |
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Join Date: December 3, 2002
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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. |
January 25, 2019, 04:25 PM | #27 |
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Join Date: October 14, 2012
Location: North Central, PA
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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 |
February 10, 2019, 10:23 PM | #28 | |
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Join Date: July 26, 2010
Posts: 273
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Quote:
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. Red |
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February 11, 2019, 01:27 AM | #29 |
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Join Date: October 14, 2012
Location: North Central, PA
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I bought a magnetospeed.
My estimated velocity was about 75 fps faster than actual. |
February 11, 2019, 07:19 AM | #30 |
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Join Date: June 23, 2013
Location: Central Taxylvania..
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Kilo,
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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February 11, 2019, 11:01 AM | #31 | |
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Location: Alaska
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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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