Calculating velocity

[RedSkyFarm]

Trying to calculate bullet velocity at 300 yds without a chrono. It is zeroed at 200 yds. I can measure the bullet drop. Was wondering if there was a formula that I could use the known distance and the drop and figure the velocity. Thanks for any help.

[std7mag]

Are they factory loads? Handloads?
What caliber?
What cartridge?
What type of bullet? Spire point? Round nose?
What type of firearm?
How long is the barrel?
What is your elevation?
What is your temp?

Ohhh, just get a chrono!!!

Seriously, for like $100 you can get a decent one.

[TXAZ]

Show up to a large range and nicely ask someone with a crono if you can try it out.

[Dufus]

I could use the known distance and the drop and figure the velocity

Sounds like the definition of a ballistics table to me.

You would need to know the G1 ballistic coefficient of the bullet to get it close.

Some load manuals have ballistics tables.

[hounddawg]

I can measure the bullet drop. Was wondering if there was a formula that I could use the known distance and the drop and figure the velocity

Only if you are capable of shooting perfect one hole groups at 300

168 gr bullet , .5 BC (G1)

drop at 300 @ 2500 FPS 17.3 inches, @2600 15.7 inches, @2800 FPS 14.3 inches, that's a 1 MOA difference, are you that accurate of a shot? If you are like most your 5 shot group will be larger than 3 inches

you might shoot 20 shots and get the average drop then play with a ballistics calculator to get a approximation

[jimbob86]

Your rifle would have to be perfectly level with the ground for every shot to calculate drop ..... or at least perfectly consistent angle in relation to the ground.....

You'll likely burn up more money in ammo getting good data than if you would if you just went and bought a Chrony.

[Jim Watson]

It is theoretically possible. There was an old Gun Digest article titled 'The Trajectory Chronograph.'

Given a perfect zero at 200 yards and the drop to group center at 300 yards, you could run one of the ballistics programs on a trial and error basis.

Given the ballistic coefficient and other starting conditions, just guess a muzzle velocity and see if it gave your drop. Guess again and iterate until you get a match. Most such programs will show the velocity along the whole range.

[higgite]

Your rifle would have to be perfectly level with the ground for every shot to calculate drop

Exactly. And your POA on the target would have to be dead level with the muzzle to give you a point of reference to measure bullet drop. Know a good surveyor? Or you could look through the barrel if you can get your eyeball or a mirror behind it. But maybe the biggest challenge is keeping the barrel perfectly level and aimed at the intended POA until the bullet exits the muzzle. Sounds like a job for a chronograph to me.

[hdwhit]

Red Sky Farm wrote:
Trying to calculate bullet velocity at 300 yds without a chrono.

You would have to be a very good shot to get your bullet through the sky screens of a chronograph set up 300 yards away. Sounds like a good way to have a chronograph die of "lead poisoning".

Was wondering if there was a formula that I could use the known distance and the drop and figure the velocity. Thanks for any help.

Yes there is. How are you at working n-th order non-linear differential equations?

Once a bullet leaves the muzzle, it begins slowing down in response to air resistance and falling towards the ground in response to gravity. Both of these responses are acceleration so to calculate the velocity 300 yards out requires knowing the acceleration and this changes as the bullet slows down.

If post-Calculus mathematics aren't your thing (it used to be mine, but I way out of practice) then your best bet is to get a chronograph, determine the velocity at the muzzle, look up ballistic coefficient from the manufacturer's website and then use:

• A set of published ballistic tables (https://guide.sportsmansguide.com/ballisticscharts/), or
• An on-line ballistic calculator (http://www.winchester.com/learning-c...alculator.aspx).

Ballistic coefficients change with velocity so what you see on the target may be a little different from what the chart predicts, but they should be close.

[444]

Download a cell phone ballistics app, plug in the numbers you know, and whatever numbers you don't know, you are going to have to guess from the information from a loading manual, or the muzzle velocity on the box of factory ammo or something like that. You are going to need to know things like the ballistic coefficent of the bullet you are using, scope height above bore etc. The more accurate the information you put in, the more accurate the numbers it spits out: Garbage in, Garbage out. You tell the app you are zeroed for 200 yards. You know your drop at 300 yards. You plug in a muzzle velocity from a manual that corresponds to the load you are shooting, you get the bullet BC from the bullet manufacturers website..................

Then you can start plugging numbers into the app until you get numbers that jive with the drop you are getting on paper. In other words, you use the app to calculate the drop at 300 yards: if that isn't the same number you are actually getting at 300 yards, then change the muzzle velocity until you come up with the same amount of drop at 300 yards that you are actually getting. When you come up with that number the velocity should be fairly accurate.

This isn't the way I would do it, but you could come up with a ball park number working backwards like this. As crazy as all this sounds, this is sort of how you actually use the app. only you are starting off with much more concrete numbers.......but, when you actually shoot, you can correct the dope given by the app to coincide with what you are actually seeing at the range. The first time I tried one of these apps, I put in good solid data initially. I had a muzzle velocity that I got from my chronograph measured over 60 rounds. I went to a range and fired at 500 yards. I was two minutes low. I corrected the app which then asked me if I wanted to correct the muzzle velocity or the BC of the bullet. I said I wanted to correct the muzzle velocity which it did automatically to jive with the two minute correction. I then immediately went to 1000 yards and dialed in the dope the app gave me (after the correction) and it was right on the money. I then went to 1200 yards and the same thing happened. What you are trying to do is only a little bit off of this beaten path.

It would make the whole thing very simple if you get an accurate muzzle velocity. If you go to any kind of club or public range you are going to run into someone that has a chronograph and I would bet they would let you take a couple shots through it so you are starting off the calculation with an accurate muzzle velocity.

[jmr40]

It is theoretically possible to calculate velocity and get fairly close. But WHY? Most people measure velocity in order to save ammo in determining trajectory. If you calculate trajectory by the trial and error method you've already burned the ammo and know your trajectory. At this point knowing speeds isn't all that important.

If you do this many times you'll save enough on ammo costs to pay for a $100 chronograph.

[briandg]

Just a hint here. What ranges are available near you? Are you in a club? There are thousands of chronographs out there and the people who use them are eager to share most of the time. If a club or range near you has a deer season sight in event, someone will have one there.

[Unclenick]

Redskyfarm,

It can be done, but is a nuisance. The problem with the basic approach is the measurement isn't very sensitive. Figure that if I shoot a .308 175 grain Sierra MatchKing bullet at about 2550 fps, at 300 yards a point of impact drop of ¼moa is about 35 fps change. That means each foot per second of velocity difference represents about two hundredths of an inch difference in bullet drop for a perfectly fired and directed bullet at 300 yards. It's just too small to be sure barrel whipping or wind lift from a cross wind aren't messing with your reading beyond how small a difference you probably want to see.

If you think you can get a long range in zero wind conditions, a method of reducing the shot count is to set up two sheets of paper as target screens so each bullet has to pass through both, say, one at 100 yards and one at 300 yards. This eliminates shooter error and group size error because you find the two holes for each bullet and then find the height difference. You do have to know your close range zero is correct, though, and that usually means averaging at least 30 shots to find actual POI to get a reasonably accurate result. The next catch is that you have to have the POA of both screens perfectly superimposed. This is usually done by setting up the long screen with a target and sighting it in with a scope that is fixed to the shooting bench, usually on a rifle with the bolt removed and in a gun vise and with the parallax correctly adjusted for the longer range, and then switching the parallax adjustment to the shorter range and having a trusting friend slip the close target into the target holder and adjust it until the scope says it is equally well centered.

Somewhere along the line, though, when considering all this complexity and added shooting, the fact that for less than $100 you can get a chronograph more accurate than you will be able to measure by this method will start to have more appeal.

[AllenJ]

Redskyfarm a drop test is a great way to confirm what a chrono and the bullet manufactures are saying but in the end it is not going to tell you your rifles FPS without knowing that the stated BC of a bullet is actually what they say it is. With that said I use drop test and like that they are telling exactly what my rifle is doing. I sight in at 100 yards and then shoot no less than 5 shots at 300 yards. I then add the total drop of all shots fired and divide it by the number fired to get a statistical average. Then I refer to Hornady's online Ballistics Calculator and see what I should be using as a velocity in my phone app.

[jonnefudge]

Totally agree with what's already been said.. But I really think it's a great question!

My advice is to get a magnetospeed.

[Unclenick]

Incidentally, per AllenJ, keep in mind that your sight settings are as fine an adjustment as you can make, so knowing velocity to the last foot per second isn't necessary for pretty good trajectory calculation for sight elevation adjustment purposes. Do a search on Art Pejsa's formula and its variations. Exact velocity differences yield some useful information about your powders, but if you just want good trajectories you don't need to go that far. A good ballistic coefficient is very helpful, though, and Bryan Litz's book, Applied Ballistics for Long Range Shooters has a lot of them for bullets he's measured the actual performance of by shooting them 1000 yards and capturing velocities at different points.

[RedSkyFarm]

Thanks for all the input. Didn't realize the extent of my question. I have located the use of a chrony. I've taken a look at some ballistics calculators (don't quite understand them yet). I'll see what I can do. Again appreciate all the information.