Home Forums Hunting & Shooting Ask The Gunwriters Effect of elevation on velocity

Had a brief discussion on-line with "my other brother Darryl" on this subject a year or so ago, but now I have some empirical data to base an inquiry on. I took my Chrony Beta to the range yesterday. The .35 Whelen loads that chronographed an average of 2546 in Colorado at about 5500 feet elevation with about 10% rH clocked an average of 2376 at about 150 feet above sea level with about 60% rH. The load is 53 gr H335, 250 gr Speer GS, WW primers and R-P .35 Whelen brass. One would be tempted to attribute the velocity difference to powder lot variations, but these cartridges are from the same loading session the others were from. I also chrono'd some .280 loads that ran about 150 fps less than ones checked in Colorado, but they were loaded with a different lot of 760.

So, gunwriter friends: Will going from a mile high to sea level and adding a bunch of rH knock five to seven percent off muzzle velocity, all other factors being equal?

(And for everyone else -- yes, I know that the loss will have no material effect on my ability to kill game with either rifle. It does have an effect on trajectory, however, and I do like to know such things.)

Thanks for your insights.

Neither elevation nor humidity should affect muzzle velocity, which is a product of solely interior ballistics. My first thought is that I wouldn't trust a Chrony as far as I could heave it. My second thought is the suspicion that you fired only three- or five-shot groups -- not enough for any meaningful comparison. If you fire ten rounds, then fire another ten rounds of the same consistent load, you're likely to find their averages different, anyway.

I also suspect that your chronograph wasn't exactly the same distance from the muzzle in both series of tests. A few feet closer to the muzzle, velocities are higher, of course.

Muledeer,

I remembered the Sierra reloading manual had a discussion of standard conditions and corrections for deviation from same, so I pulled it off the shelf to check (Sierra Rifle Reloading Manual, 4th ed.), p611ff.

Sierra follows the US Army's Aberdeen Proving Grounds standard conditons: sea level, 29.53" Hg or 750 mm, 59 Deg F and 78 percent relative humidity. (I am glad I live in the West; for me the East Coast is the West Bank of the Mississippi River <img src="/ubbthreads/images/graemlins/wink.gif" alt="" /> ).

They note that the primary environmental variable affecting MV is the temperature, while the other factors affect the speed of sound in air and affect drag, BC and hence trajectory. Altitude is a major factor (perhaps a 30% change in BC) while relative humidity is minor (.25% to .5% change).

As I recall the correction to muzzle velocity from Chronograph velocity (at the mid-point of the screens) is small, about 1 fps per foot.

With all that mumbo-jumbo considered I expect temperature is the major factor at the muzzle.

jim dodd

Will the "clock of the chronograph" (oszillator) or other electronics be affected by temperature-, humidity- or barometric changes ?

I have used a Chrony and other chronographs during deliberate tests from 0 F. to over 100 F., mostly to test some of the newer powders that claim not to be very affected by temperature. Got almost exactly the same readings at 0 as at 70 with Hodgdon Extremes and some Ramshot powders, so apparently chronographs aren't terribly affected by low temps. (Did warm the battery in my pocket between strings, however.)

Have found many older powders are very much affected by cold, as much as 2 fps per degree F. or even more. Even the newer powders are affected by heat, though not as much as many older powders. Just leaving a box of ammo in the sun for a few minutes can drastically affect MV.

Suspect that my other brother Darryl's different readings at elevation and sea level are due more to the ammo's temperature than any other reason. A lot would depend on the powder used.

JB

A Chrony can be manipulated to change velocities inadvertantly or on purpose. The Chrony is a fold up unit that has to be laid out perfectly flat to get consistant readings. I remember one archery shop in this neck of the woods who claimed the bows he was selling were faster than the bows of any competetor and he would prove this to anyone by shooting arrows over a chrony and there it was, proof that his bows made the velocity he claimed. He sold a lot of bows this way. Only problem was when the customer took his new bow out and shot it over someone else's chrony it was slower than it was in the store. It was discovered that the shop was not opening up the chrony all the way flat and therefore the windows where the photosensors are did not have a parallel view and the higher over the machine the projectile was shot the faster the reading would be.

I've learned to always make sure my chrony is always flat. I used a straight edge to make sure it is flat and marked the machine so I can return it to the same position each time I use it. Chronys can also suffer from mis-aligned photosensors and thats how I figure the variation in readings sometimes from one unit to another. Even if your chrony is not calibrated perfect, you can get consistant comparison readings from lot to lot by ensuring your chrony is opened to the same degree each time.

Good point! I should have mentioned the necessary "flatness" of Chronys as well. But I have been using them (and other chronographs so long) that I assumed everything would be set up right.

Still, it would take an awful lot of misalignment to get the differences in velocity cited. I have tested Chronys opened fully flat against the same Chrony apparently but not quite flat, and the difference was never more than 50 fps, and usually less.

JB

For the most part as long as it is opened up to within a few degrees of flat and you shoot through a window that isn't excessively high, the readings you get will be somewhere in the ballpark. I've seen one that was over opened at the range one day with the hinge bent by about 20degrees over, most deformed, giving readings that were somewhere around 2100fps for a factory loaded 30-06 with 165 grs., and the guy was disguisted with his gun. I mentioned his deformed chrony and that if we taped it so that it was flat he might get a better reading. Alas, it read just over 2800 for a 22inch barrel. He was happy. I find it hard to believe some people are so hard on their equipment.

One weakness of the Chrony is the placement of the threaded hole for tripod mounting. On my tripod, at least, the thumbscrew to tighten the attachment spans the joint between the two sections. This can lead to the Chrony not getting opened all the way, or you may push it partway shut while tightening the screw.

I always make sure it's all the way open before putting it on the tripod, and double-check after tightening.

John