I have been doing load development for my .338-06. I have found a good load. It gives 2590 fps according to the chronograph.

The chronongraph is placed 20ft from the muzzle due to practice area considerations - ditch etc.

How do I correct this velocity at 20ft from the muzzle to a 'true' muzzle velocity?

Is there a formula?

I am guessing the discrepancy is 20 or 30fps, but this is pro-rata comparing the mv and 100 yard velocities.

This program will guess at it. Put in the Chrono distance and velocity. The make sure your minimum distance is 0. It says 8.9fps diff. That would change a touch with bullet.

I have never worried about correcting. 9/2590 is .3%. I bet my Chrono is not that accurate.

Sorry it would have helped if I had posted the link.
http://www.eskimo.com/~jbm/ballistics/traj/traj.html

Robthom, you're on the right track. Just look at a table to estimate the velocity loss from muzzle to 100 yards (300 feet). The your fraction is then 20 ft divided by 300 ft. 20/300 = 1/15 or the bullet lost one fifteenth of its velocity in the first 20 feet.

Like ramrod says, the correction is not really worth the trouble.

KenO

Using the link that ramrod340 supplied set the "range increment" to 7 (yards which is 21 feet and your velocity to the "muzzle velocity".

Slowly increase the muzzle velocity until the velocity that you saw on your chronograph matches the first incremental velocity.

When your chrono velocity matches the first incremental velocity you'll know the muzzle velocity.

quote:

Using the link that ramrod340 supplied set the "range increment" to 7 (yards which is 21 feet and your velocity to the "muzzle velocity".

Slowly increase the muzzle velocity until the velocity that you saw on your chronograph matches the first incremental velocity.

Frank it is easier than that. When the program is asking for muzzle velocity it really means velocity at the chrono. So if he inputs his 2590 and the chrono distance of 20' in front of the muzzle and then sets the minimum range to 0 the the results will show velocities starting at 0 (muzzle) out to what ever distance he has as max. I ran it using his data and a 225gr that is how I got the 8.9fps

As mentioned above, it's not really worth the trouble trying to figure it out. If you want a rough estimate then for most centerfire rifles it's around 1 fps per foot, so if your chrono is 20' away then you lost around 20 fps.

its a 338-06! its obvious you aint shooting for the 1000yard world record. just round it up to 2600fps and get out there hunting.

Thank you gentlemen!

My reasons for asking were:

1) getting some idea of drop at ranges other than 100 & 200 yards.

2) bureaucratic / licencing reasons here in the UK concerning public ownership of firearms.

I have easy access to 100 & 200 yard ranges through my rifle club membership. So I can check the zero / drop performance. I would also like an idea of the trajectory.

So, I was wanting to run a ballistics program and then compare the observed / "measured" drop with the calculated drop at these control distances. This would provide more confidence in the calculated drop at 250 & 300 yards.

The programs ask for mv, hence the question. I am trying to reduce the errors as much as possible. I have been using the Norma web page ballistics program so far.

Secondly, the UK police / state are not sympathetic to shooting. The authorities seem only to know what is in "their book".

To them, as I found out when renewing my licence last Summer, there are only two .338 rifles: .338 Win Mag and the .338 Lapua Mag. They will not allow you to have expanding ammo for hunting for these as there is no legitimate use in the UK. According to them.

So, I have to demonstrate to the authorities my .338-06 is not a .338 Win Mag, .340 Weatherby or .338 Lapua Mag etc - does not have same / comparable performance. I am allowed to have expanding bullet heads for the .338-06 for stalking / hunting deer.

Returning to a technical slant, I was unsure of the velocity drop.

Knowing resistance is proportional to velocity, among other things like S.D., nose form / ballistic coefficient etc, I wondered whether the relationship was:

linear, my first assumption with no other information / lack of other data

or some kind of inverse power rule

It seems my initial assumption is good enough for practical purposes.

You'll have more variance between shots than the correction. Unless every one of your shots is exactly the same velocity the point is moot.

quote:

Originally posted by robthom:
..I have easy access to 100 & 200 yard ranges through my rifle club membership. So I can check the zero / drop performance. I would also like an idea of the trajectory.

So, I was wanting to run a ballistics program and then compare the observed / "measured" drop with the calculated drop at these control distances. This would provide more confidence in the calculated drop at 250 & 300 yards. ...

Hey robthom, By that do you mean you will not be doing any practicing beyond 200yds and just rely on the 'Calculated Drop" when Hunting?

The difference in 200 fps velocity, 2600 vs 2800 fps with a .338 cal 225 gr partition at 350 yd useing a 200 yd zero is about 2" difference in drop, 16.9 vs 14.3 according to my RCBS load program. The little you get from ajusted muzzle velocity or and extra gr of powder won`t mean squat at most hunting ranges.
I wouldn`t sweat it.

Hot Core,

That would be a fair inference, however, it is not my intention.

I hope to back it up at circa 250 & 300 yards. However, I do not have so regular / easy access to 300 yard ranges. At that distance, friendly farmers and over crowded island and all, I am pushing it a bit.

So, I will not be getting the amount of practice at those ranges as I would like. Additionally, at longer ranges both operator error and the wind start affecting your tests.

I need some sort of reference / yard stick for comparison and ballistic programs provide some semi neutral information to compare to my rifle's preferences.

Nonetheless, I am not trying to turn my .338-06 into a long range target rifle. Occaisionally though, we all need the knowledge to shoot at longer ranges - 250 yards to stop a 'runner'.

quote:

Originally posted by robthom:
That would be a fair inference, however, it is not my intention. ...

Hey robthom, Good for you. It is a bad practice to calculate the Drop Rate and never actually shoot it at those distances - which you obviously know.

quote:

...at longer ranges both operator error and the wind start affecting your tests.

Yes indeed. I do understand how difficult it can be in some places to locate a Range with adequate length.

It is amazing how much more concerning a persons individual shooting technique and ability to read the environment can be learned at 400yds than it can at 100yds(or less).
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As long as you realize the Bullet begins falling at a "Faster Rate"(aka dropping like a rock) the farther it gets from you, it will help hold in tendencies to just Blast Away way out yonder. Somewhere around here I have old graphs I used to make with a "French Curve" just to give myself Reality Checks. That was l-o-n-g before you could just plug it in some razzle-dazzle software and let it do all the Plotting and Printing. Even the really fast Bullets drop quickly once you get out to 400yds or so.
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By the way, Ol' Joe's last post says a WHOLE bunch. If for some reason you want the Muzzle Velocity to be more than what you are getting at your current 20', just add whatever amount of Velocity makes you grin , 50fps, 100fps or even 200fps, because (from my perspective) it really doesn't make a quack.

Some of the real fancy chronographs even have you add a "Fudge Factor", as I've been told by a guy that should know. That does seem to help some of users get indications that they really want to see.

Best of luck to you.

Thanks Mr. Oehler for your response to the question, coming from you I know we can count on it. I treasure my 35P Oehler chronograph each and everytime I use it. Good shooting.