True MV vs FPS at chrono distance

[Nature Boy]

For you guys that don’t have a labradar, what adjustments do you use to get your MV when your chrono is at 15’?

 

[Bc'z]

None, I'd be more interested in knowing velocities at known distances tho.
100-500 yards. But the thought of setting my chronograph up in front of a target makes me nervous.

 

[Nature Boy]

My reason for wanting to know is to compare my velocity vs what others are getting who may be using the labradar. Ballistics programs are correcting for chrono distance. I thought there might be a simple calculation or rule of thumb.

At the end of the day it’s really not that important. Evey rifle is a discrete system

 

[Ned Ludd]

The LabRadar does not measure velocity at the muzzle; it also back-calculates V0. The only device I know of that actually measures velocity right at the muzzle is the MagnetoSpeed. Here is a link to a recent simultaneous test of 3 different chronographs carried by a forum member:

http://bulletin.accurateshooter.com/2018/06/chrono-camparison-test-labradar-magnetospeed-oehler-35p/

For the purpose of back-calculating MV based on a chronograph reading at some distance out from the muzzle, using a ballistic calculator such as JBM Ballistics is fine. Simply set the program to give you velocities at one yard increments out to 15 or 20 yd. Then change the MV input manually until the velocity at the distance your chronograph was set in front of the muzzle exactly matches what the chronograph measured. Depending on the distance you use, you're typically going to be looking at a value from about 8-12 fps faster than the actual chronograph reading. Any discrepancy between the back-calculated value and the true value isn't worth considering; you can't shoot the difference, if any, and it will generally be much less than your ES value for 5+ shots.

 

[divingin]

The only device I know of that actually measures velocity right at the muzzle is the MagnetoSpeed.

That's reading a couple/few inches out as well, isn't it? Closer, granted, but still not reading the speed at the crown?

FWIW, it's not really the muzzle velocity that's important (my opinion, of course, and talking about a single rifle), but a set of readings at largely similar conditions. Add in the fact that a chrono is generally accurate to a couple of percentage points at best, and you can easily compare apples to oranges and not be off by very much.

In OP's case, where he's trying to compare different rifles, loads and chronos, I would hazard a guess that it's largely going to be a lost cause, at least in any meaningful way. Too many variables: Chrono accuracy, estimation of chrono distances, poder scale calibrations, temps, altitudes, etc. I would personally just take the numbers at face value and run with that (then again, I have an aversion to doing unnecessary math. YMMV.)

 

[alinwa]

What's fun is shooting over multiple chrony's. I've a LabRadar, three chrony's mounted to a board and an Oehler 43 to shoot thru.....

And oddly enough I couldn't give a rat's ass what my actual muzzle velocity is. Never even entered my head to wonder.

 

[Ned Ludd]

That's reading a couple/few inches out as well, isn't it? Closer, granted, but still not reading the speed at the crown?

Yes. Perhaps 2"-4" out, depending on exactly where it's mounted. IMO, not a distance worth consideration when compared to the 10' to 15' that other types of chronographs are mounted in front of the rifle. But you're correct, the MS is technically not mounted right at the muzzle, either.

 

[Nature Boy]

Duh

It just dawned on me to check the ballistics table on one of my saved loads.

Sure enough, it gives the calculated MV at 0’

In this example, my 223 90g VLD was 2,695 at 15’ and the table shows 2,701 at 0’

Exactly what @Ned Ludd said above

 

[marazzi59]

Check out the applied ballistics website by bryan litz. There is a great rite up and chart for this. And yes if you're shooting at any great distance it matters.

 

[Kings X]

I put my 35P up against my target at 200 yards.
Then enter data in Ballistics AE telling it my chronograph is at 600 feet.
I ask for Minimum Range to be “0”.
It does the conversion.

 

[Lou Murdica]

Alinwa is right on.
I shoot over the 43 for testing different powder comparison. As far as loads for my rifles short range I want small group in two different nodes. My long range
Best Groups at upper node .

 

[spife7980]

You can back into it http://www.appliedballisticsllc.com/Articles/ABDOC121_VelocityDecay.pdf

 

[CharlieNC]

What's fun is shooting over multiple chrony's. I've a LabRadar, three chrony's mounted to a board and an Oehler 43 to shoot thru.....

And oddly enough I couldn't give a rat's ass what my actual muzzle velocity is. Never even entered my head to wonder.

Is it the year of the rat? They get blamed for everything.

 

[mikecr]

A calc spreadsheet for this:

 

[bbarber25]

Check out the applied ballistics website by bryan litz. There is a great rite up and chart for this. And yes if you're shooting at any great distance it matters.

What he said ^^^

 

[Debbie Odendaal]

Hi all new to forum i live in South Africa and am looking to buy the LabRadar they dont stock it here any ideas please

 

[Mozella]

For you guys that don’t have a labradar, what adjustments do you use to get your MV when your chrono is at 15’?

I don't know about chronographs designed many years ago that only display MV at the chrono, quite a few of which are still in use, but modern recording chronographs usually have at least an input for the distance from the muzzle to the chrono so that you can get MV at range zero. Good ones let you also enter some ballistic data like BC, bullet weight, etc. in order to calculate an even more accurate estimate of the MV at range zero. The Labradar is forced to do that calculation since the actual measurement is done quite a distance downrange, so extrapolating the actual measured velocity to get MV at range zero is mandatory.

If you have no other way to figure it and if you place your chronograph 15feet in front of your muzzle, you can add 6 or 7 fps to the chrono reading and you'll be close enough to compare your performance with that of someone else.

 

[dmoran]

Hi all new to forum i live in South Africa and am looking to buy the LabRadar they dont stock it here any ideas please

I suggest to go through Bullet Central (https://bulletcentral.com). The owner (Chris Harris) is originally from S. Africa and does a lot of business there, and around the world.