Calculating velocity

[mroels]

Anyone know the equation to calculate velocity if you know POI at different ranges and BC of the bullet, assuming zero at 100 yds. I know it would be a guesstimate at best but still curious since I don't have a chrono.

[Munk]

Anyone know the equation to calculate velocity if you know POI at different ranges and BC of the bullet, assuming zero at 100 yds. I know it would be a guesstimate at best but still curious since I don't have a chrono.

Without a very precise measurement of the angle of your rifle's incline, you can't really math it out very easily.

If you know the rifle is 100% horizontal, you can use an estimator for gravity to figure out how long it took to fall to the POI during flight. This will give you a time measurement. For example, if it's hitting 1 inch below a perfectly horizontally aimed shot wolfram alpha tells me this is .072 seconds.

This means that the distance of your shot is covered in .072 seconds.

A 100 yard shot covered in .072 seconds. 300ft/.072seconds = 4166ft/s.

This is absurdly difficult to make useful though, any miniscule angle of incline can make this type of estimation worthless. Almost no scope will be parallel to the bore, so you can't even trust where it's aiming once you've leveled your rifle.

[HighLander51]

you can use an estimator for gravity to
This means that the distance of your shot is covered in .072 seconds.

A 100 yard shot covered in .072 seconds. 300ft/.072seconds = 4166ft/s.

You can 'estimate' that acceleration due to gravity is 32 feet per second per second because that's what it is. Heavy bullets fall at the same rate as light bullets, but they travel at different velocities. A .223 Rem goes 2 times as fast as a 45-70. What kind of a rifle develops 4,266 ft/s?
Chronos are cheap, just buy one.

[BrianRodela]

If you have the Shooter app, you can work it in reverse. What is your Dope for the different distances and I can do it for you.

[CalTeacher]

Seriously, just buy a refurbished chrony. They are inexpensive and worth the investment to know exactly what your loads are doing.

[Munk]

You can 'estimate' that acceleration due to gravity is 32 feet per second per second because that's what it is. Heavy bullets fall at the same rate as light bullets, but they travel at different velocities. A .223 Rem goes 2 times as fast as a 45-70. What kind of a rifle develops 4,266 ft/s?
Chronos are cheap, just buy one.

few rifles on a completely horizontal trajectory will only drop 1" at 100 yards. Most drop more than that. the 1" was used for simplicity of example. Very few rifles will hit over 4000 FPS.

Even your .223 will drop almost 2" at 100 yards, and the slow lumbering .45-70 will drop much farther. Neither of which contradicts my loose estimator.

[mroels]

If you have the Shooter app, you can work it in reverse. What is your Dope for the different distances and I can do it for you.

Yeah I've been doing it in reverse on the handloads.com calculator. Which Shooter app are you using?

[BrianRodela]

Shooter

[mroels]

Shooter (just want to make sure its the right one before dropping $10) by Sean Kennedy on apple?

[BrianRodela]

Yup, thats it, great program. Got it for the Ipad!!

[mroels]

Without a very precise measurement of the angle of your rifle's incline, you can't really math it out very easily.

If you know the rifle is 100% horizontal, you can use an estimator for gravity to figure out how long it took to fall to the POI during flight. This will give you a time measurement. For example, if it's hitting 1 inch below a perfectly horizontally aimed shot wolfram alpha tells me this is .072 seconds.

.

Again I realize this is wildly innacuracte but I'm a curious guy, and by that I mean both weird and interested.

Figuring out the incline isn't too hard albeit not super accurate. You have all the info e.g. Length of barrel, height at the muzzle, height at the chamber.

But let's assume my rifle is 100% level and drop at 100 yds is 1" what is the formula you're using to get .072 seconds?

Is it:

Time = square root of 384"/gravity

[mroels]

Seriously, just buy a refurbished chrony. They are inexpensive and worth the investment to know exactly what your loads are doing.

A chrony is my next firearms related purchase but I just want to understand the math. Yeah I'm a nerd

[mroels]

Yup, thats it, great program. Got it for the Ipad!!

I have been using the iStrelok app which is nice because it'll also show your holds on your specific scope's reticle, assuming you've got a main brand scope. And the Gun Log +P app to build a data book.

[CGT80]

I have the CED M2 chronograph. I have only used it about 4 times in the last year and a half, but I have really enjoyed working up some loads and knowing what is actually happening with them. The M2 is around $200 but will do just about anything you want while at the range. I download all the data to my computer and copy and paste it into my reloading data that I keep in excel. The electronics are separate from the sensors, so it is easy to manipulate the controls while standing 10-15 feet from the sensors.

The velocity of your ammo through your gun likely won't match what factory ammo or reloading books list. You might get close by using their info as a baseline to do your calcs, but it will be one more variable to trying to figure this out without a chrono.

Have fun.

[Munk]

Again I realize this is wildly innacuracte but I'm a curious guy, and by that I mean both weird and interested.

Figuring out the incline isn't too hard albeit not super accurate. You have all the info e.g. Length of barrel, height at the muzzle, height at the chamber.

But let's assume my rifle is 100% level and drop at 100 yds is 1" what is the formula you're using to get .072 seconds?

Is it:

Time = square root of 384"/gravity

As mentioned in my post, I used wolfram alpha to calculate it for me. It uses the formula T = Sqrt((2*h)/g)

Since we know how far it fell (my example of 1".) and we know what gravity is supposed to be, we can get an estimated time to impact because it's falling the entire time it's in flight. Since we now know the time, and we know the range we were shooting at, just divide the number of feet traveled, by the number of seconds it took to impact and you have a FPS that is an average for its entire flight.

Now that i bother to think about it, we can eliminate the worries about angle at time of shot by using a bore-laser to find where it's pointed exactly, and then measure down from that point to the POI. Substitute this measurement for my 1" and you'll get a new time to work into the simple FPS model. (This'll be borked by extreme angles, but for something that's mostly horizontal, the difference is negligible.

If you like, you can get a muzzle velocity by working backwards with the BC, but I think the flight average is useful enough.

For another example, I put 2" into wolfram, it spit out a time of .1 (unexpectedly simple and easy to work with). 300ft/.1second = 3000 FPS average speed on a 100 yard shot. Based on what I know of .223, this is damn accurate.