Formula for calculating muzzle velocity based upon bullet speed down range?


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Onewolf
October 2, 2013, 08:42 AM
I was wondering if anyone has the formula for calculating the muzzle velocity based upon the bullet speed measured by a chronograph at 15' from the muzzle? I have searched but I cannot find it.

Thanks.

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NeuseRvrRat
October 2, 2013, 09:26 AM
kinetic energy = 1/2*mass*velocity^2

Onewolf
October 2, 2013, 10:28 AM
I don't see how that equation helps answer the question I asked.

If my chronograph says the bullet speed was 2900 FPS 15 feet in front of the muzzle, what was the bullet speed as it exited the muzzle? 2900 fps + ?

NeuseRvrRat
October 2, 2013, 10:32 AM
in 15 feet, there's not enough difference to matter

and for some reason i read muzzle energy instead of muzzle velocity in your first post. i apologize.

mdm
October 2, 2013, 10:40 AM
Are you going to also consider temperature, humidity, atmospheric pressure, elevation, wind direction/speed, direction of bullet flight (you've got to consider the earth's rotation)?

I'm being a bit sarcastic here, but is the drop in fps in 15 feet is really going to matter?

MErl
October 2, 2013, 11:14 AM
The drag on a supersonic bullet is not a simple thing to work with. Just assume 15' is the same as muzzle because your sigma will be much greater than delta.

Otto
October 2, 2013, 11:34 AM
Using a formula would be a very inaccurate way to calculate velocity (at any distance)....that's why we use chronographs.

Haxby
October 2, 2013, 12:00 PM
Go to a ballistics calculator like the one at handloads.com. Set intervals at 5 yards. Estimate B.C. Take a guess at the muzzle velocity to get the number you want at 5 yards. Adjust as needed.

Carl N. Brown
October 2, 2013, 12:30 PM
I think the difference in velocity between muzzle and 15' is pretty close to the difference in muzzle velocity of individual rounds a few feet per second if that much.

(you've got to consider the earth's rotation)

If you're using a Mark 37 Fire Control Director with a Ford Mark I Ballistic Computer or later, for 5" naval gun or larger, over a nautical mile in range, yep you do have to consider earth's rotation in plotting POA. For most of us, the difference is within the normal dispersion of shots at practical ranges.

Andrew Leigh
October 2, 2013, 01:47 PM
Hi

You don't mention calibre, bullet type so I have made a couple of assumptions.

.308
150gr SPBT Game King

2 900fps @ 15' will result in 2 911 at the muzzle.

Ridgerunner665
October 2, 2013, 01:47 PM
Most rifle rounds that I have checked...lose 6 fps in 10 feet...so 15 feet would be real close to 9 fps.

mdm
October 2, 2013, 01:57 PM
If you're using a Mark 37 Fire Control Director with a Ford Mark I Ballistic Computer or later, for 5" naval gun or larger, over a nautical mile in range, yep you do have to consider earth's rotation in plotting POA. For most of us, the difference is within the normal dispersion of shots at practical ranges.
Yep - and for most of us the speed of the bullet at the muzzle equals the speed of the bullet 15 feet later.

And then there is this:
http://www.magnetospeed.com/

Onewolf
October 2, 2013, 01:57 PM
So basically none of y'all know the formula/equation I am looking for. (Regardless of why I want to know the formula). :D

Some interesting answers none the less. I especially like the one about how ballistic formulas are inaccurate and that's why people use chronographs. Really? I find my ballistic (smartphone) apps to be incredibly accurate.

ArchAngelCD
October 2, 2013, 02:12 PM
So basically none of y'all know the formula/equation I am looking for. (Regardless of why I want to know the formula). :D

Smiley face or not it's a little messed up you criticizing the people on this forum for not knowing something you don't know. Unreal... :rolleyes:

Haxby
October 2, 2013, 02:18 PM
I have the formula, I copy it, but it won't paste here.

gamestalker
October 2, 2013, 03:27 PM
I'm not sure anyone here will be so helpful in the future, considering that you've insulted their helpful opinions.

Calm down a bit, it's ballistics and a lot comes into play, thus a lot to be considered that you failed to include in your original post. This is intended to be an enjoyable hobby, not one of conflict and confrontation.

GS

jr_roosa
October 2, 2013, 05:14 PM
The JBM calculator will give it to you. There is a variable for chrony distance and it back calculates MV.

J.

DanTheFarmer
October 2, 2013, 06:28 PM
Make your own formula. Go to any ballistics calculator (I use the one at www.hornady.com) and get the interval down as small as it will go. Say that is 50 yards. 15 feet is 3 yards. The calculator will give you the velocity at the muzzle and at 50 yards. Assume the velocity loss is constant over that 50 yards. It isn't but it is a very good approximation and easy to calculate. Say the velocity loss is 100 fps. So 100 fps lost in 50 yards = x lost in 3 yards, 100/50 = 2 fps/yard, therefore in 3 yards 6 fps will be lost.

I hope that is clear.

Dan

Clark
October 3, 2013, 01:32 AM
Each bullet loses velocity at a different rate at a different velocity.

Quickload has a large library of bullets.
It can calculate anything like that in seconds.

As an example I used it to calculate the distance it takes for different 7mm cartridges to slow down to the same velocity, given a 26" barrel, Hornady moly coated 162GR BIG GAME POLY CARB TIP 28452 bullet, and loaded to 65,000 psi with an optimized powder:




7mmT/CU ........................26.1 gr H335 2330 fps -471 yards
7-30 Waters ........................30 gr W748 2456 fps -395 yards
7mmBR ...........................31.2 gr W748 2510 fps -360 yards
7mm IHMSA .......................40.2 gr Re17 2704 fps -247 yards
7mm-08 .......................40.6 gr IMR4895 2789 fps -198 yards
7mm-08AckleyImproved .........43.6 gr Re15 2833 fps -171 yards
7x57mm ...........................44.6 gr Re15 2843 fps -165 yards
7x57mmAckleyImproved .........46.4 gr Re15 2873 fps -150 yards
284Win .........................50.0 gr IMR4350 2890 fps -172 yards
280Rem ............................53.5 gr H414 2961 fps -152 yards
7mm Rem short action ultra mag 55.5 gr Re17 3015 fps -73 yards
280RemAckleyImproved ........57 gr IMR4350 3019 fps -71 yards
7mmRemMag ......................68.2 gr Re22 3129 fps -15 yards
7mmRem ultra mag ............83.3 gr IMR7828 3130 fps -15 yards
7mmWeathMag ....................71.8 gr Re22 3170 fps +7 yards
7mmSTW .............82.2 gr Ramshot Magnum 3158 fps 0 yards


That is with a boat tailed heavy bullet.
With a square tailed light bullet, the distances would be closer together.

1SOW
October 3, 2013, 10:03 PM
As said above---Different bullets' ballistic coefficient changes the rate of speed loss.

An approximation can be made that will be closer with some bullets than others.

You're very welcome.

Rule3
October 4, 2013, 12:04 AM
So basically none of y'all know the formula/equation I am looking for. (Regardless of why I want to know the formula). :D

Some interesting answers none the less. I especially like the one about how ballistic formulas are inaccurate and that's why people use chronographs. Really? I find my ballistic (smartphone) apps to be incredibly accurate.

"There is an App for that":rolleyes:

The difference between muzzle and 15 ft with all the variables. is probably insignificant.Not only bullet, temp, humidity but what about caliber and length of barrel???? (the big one)

racerbob
October 26, 2013, 07:25 PM
Get a Magnetospeed chrono and read the actual muzzle velocity at .5 inch from end of the barrel.

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