One more trajectory question for you.If you sight in at sea level,then recheck your zero at high altitude,should the point of impact be the same?If not why?Would any change be due to a change in muzzle velocity,or would it be due to other factors such as thinner air or a change in the effect of gravity.?

Thinner air is one of the factors. Run some number through this Calculator and change the altitude.

Check it out here.

Already did,but another fellow insists that the difference in trajectory is due to the fact that the muzzle velocity will be higher at higher altitude.This same individual also insists that you do not need to know the sight height to use the trajectory calculations.

Gravity is less at a distance fromn the center of the earth and air resistance is lessened and as a result the tradgectory will be somewhat flattened.

You got to go a very long ways up to actually see a big enough difference to get excited about.

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This same individual also insists that you do not need to know the sight height to use the trajectory calculations.

Well that tells me that your buddy doesn't really know what he thinks he knows.

Assume that your crosshair is level with your target. The barrel is below the crosshair to hit the same point the bullet must rise and will cross the line of the crosshair twice once on the way up once going down. The higher the scope the steeper the angle must be for the bullet and crosshair to cross and the longer the distance before the bullet again crosses the line of the crosshair on the way down..

As to higher muzzle velocity I have never tested that one. Thinner air and less gravity will reduce the drop a little and velocity loss. Adding 3-5' to your point blank range at 5000 vs 0'. So your bullet will be a touch higher the full flight.

I've always thought, perhaps in error, that the difference in altitude and temperature between where I shoot and where I hunt Elk will, to some unknown extent, offset each other.

Especially with powders like RL22 or RL19, the velocity will be lower when traveling from 90 degree temps to 40 degree temps and thus make the trajectory less flat, but the difference in altitude between 200' and 7500' will make the trajectory more flat. To what extent they offset each other

Texas / 90 degrees / 200' alt / 200 gr 300 win mag @ 2900 fps / drop @ 500 yds = 37.66"
Colorado / 40 degrees / 7500' alt / 200 gr 300 win mag @ 2840 fps / drop @ 500 yds = 37.60"

Now this is not empirical evidence since I have never shot over a chrony in Colorado.

Since group size is usually directly affected by velocity, the golf ball size group you developed may be softball size by then also.

Hell, just go hunting and get as close as you can. I do know that the shot I put on that Elk last year at 253 yds went exactly where it was supposed to.

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Originally posted by ramrod340:

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This same individual also insists that you do not need to know the sight height to use the trajectory calculations.

Well that tells me that your buddy doesn't really know what he thinks he knows.

Assume that your crosshair is level with your target. The barrel is below the crosshair to hit the same point the bullet must rise and will cross the line of the crosshair twice once on the way up once going down. The higher the scope the steeper the angle must be for the bullet and crosshair to cross and the longer the distance before the bullet again crosses the line of the crosshair on the way down..

As to higher muzzle velocity I have never tested that one. Thinner air and less gravity will reduce the drop a little and velocity loss. Adding 3-5' to your point blank range at 5000 vs 0'. So your bullet will be a touch higher the full flight.

Please tell your buddy to do himself and you a favor...to shut his mouth and take some books and have a nice and enlighting reading on exterior ballistics

That's a wonderful program but the way I see it the program makes you pretend you resited your gun at say 8000 ft. So if you site for a 250 yd zero at sea level and you went to 8000 ft you'd likely hit an inch or so high at 250 which would change the down range numbers pretty good. Many times people go on a hunt after zeroing at low altitude and never resite before the hunt.
It throws off the long range shooting quite a bit unless a program can make up for not resiting in. I do realize it's kind of splitting hairs and a good reason "flatlanders" should practice where they hunt or they are on a big guessing game which isn't fair to a wonderful game animal.

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Originally posted by stubblejumper:
Already did,but another fellow insists that the difference in trajectory is due to the fact that the muzzle velocity will be higher at higher altitude.This same individual also insists that you do not need to know the sight height to use the trajectory calculations.

I'd get advice from different buddy or get another buddy. Both of those statements are WRONG.

Internal ballistics (Muzzle velocity) are not affected by elevation, air temperature, etc. External ballistics (trajectory) are.

Use the JBM program and trust the results.

Ray

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That's a wonderful program but the way I see it the program makes you pretend you resited your gun at say 8000 ft. So if you site for a 250 yd zero at sea level and you went to 8000 ft you'd likely hit an inch or so high at 250 which would change the down range numbers pretty good. Many times people go on a hunt after zeroing at low altitude and never resite before the hunt.

You lost me. Maybe it will make sense in the morning after coffee. Doesn't telling the system to zero at max PBR take care of your issue? You have the starting PBR and bullet path. At the higher elevation it will adjust itself for less drop and give you a new PBR and trajectory?

If I don't set up my scope to give me the suggested impact point for max PBR I adjust the kill radius so that the impact at 100ys is what I zero at. Then adjust the altitude. You will find that the 100 point normally stays within your group size but down range will change.

Well like a couple of people have said here your buddy needs to bone up on his ballistics a bit.

I stumbled across this as a young man about to go on his first hunting trip in the Rockies. Glad I did. I believe that it made the difference of getting an animal or missing. One shot at 500 yards collected me a nice Muley. My Uncle couldn't believe it. Probably would have shot high we were very near 8000 ft.

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Originally posted by kraky:
That's a wonderful program but the way I see it the program makes you pretend you resited your gun at say 8000 ft. So if you site for a 250 yd zero at sea level and you went to 8000 ft you'd likely hit an inch or so high at 250 which would change the down range numbers pretty good. Many times people go on a hunt after zeroing at low altitude and never resite before the hunt.
It throws off the long range shooting quite a bit unless a program can make up for not resiting in. I do realize it's kind of splitting hairs and a good reason "flatlanders" should practice where they hunt or they are on a big guessing game which isn't fair to a wonderful game animal.

I agree with this. I've had that problem several times. I live at sea level and have hunted AZ and Wyoming. The zero changes from sea level to higher altitude. Every time I hunt at altitude, I need to resight my rifle. If I recall correctly., it hits lower at high altitude.

Velocity decrease in a rifle bullet is mostly due to air drag. The veloity loss from air drag is something like 56times greater than velocity loss due to gravity.
The % of sea level air density is about .94 at 2000ft, .88 at 4000ft, .83 at 6000, .78 at 8000ft and .73 at 10000ft (at the same temp). Air density also decreases with temperature and barometric pressure. The differences aren't all that great, a 180gr 30cal bullet at 2700fps at 300yds will have about a 1/2 Minute difference vs Sea-Level and only 7/10 minute vs a Vacuum (Understanding Firearms Ballistics by Rinker p. 157).
There are also other things to consider such as velocity decrease due to lower temperatures (especially if you don't use extreme powders), etc.
So it's not a huge difference and there are lot's of variable's, but if you add up an altitude difference and an uphill or downhill angle you could end up with a couple of minutes difference. Maybe on a long 3-400yd shot it could add up.
I'd think the best idea is to double check your zero where you are going to hunt.................DJ

This is one of the biggest reasons so many (who are prepared to take a poke at longish ranges) start out with a 100 yd zero. A 250-300 yd zero (much less dope at longer ranges) can change quite dramatically going from say, 0 to 10,000 ft. But at just 100 yds, the difference due to the thinner air is still usually less than a click--pretty much negligable.

So a dropchart that has been adjusted to 10,000 ft will show fewer "clicks up" to be on at 300 yds and have a much better chance of being "on" right off the bat. It's a good idea to check when you get there and if you do try and do it at the longest range possible. Using the above method, chances are you'll be much closer when you check and this leads to less head scratching and a higher confidence level.

The trajectory of the bullet at higher altitudes will exhibit a bit less drop at a given yardage due to the thinner atmosphere slowing the bullet less quickly. There is NO (measurable) difference in bullet velocity due to altitude alone, although the typically cooler tempertures at altitude might cause a small reduction in muzzle velocity, just as lower temperatures would at sea level. The difference in gravity can barely be measured by the most sophisticated instuments, and can certainly not be identified in the trajectory of a bullet over a few hundred yards. All told, the trajectory difference between sea level and altitued is, for practical purposes, deminimus.

Zero changes between "home" and the "mountains" is usually a result of changes in humidity absorbed or evaporated from the wood stock changing its bearing on the barrel and thus altering the zero. That, and the chance of an unintended blow during transport moving the socpe, is why it is desireable to rezero after reaching your hunting destination.

If you want to see a real difference in sea level and altitude, you'll have to go to an altitude of about 240,000 miles: The surface of the moon with its 1/6 Earth gravity and no atmosphere negates the need of spitzers entirely.

This is right up there with the 30-06 vs. 270 argument. I've hunted in the mountains where barometric pressure and temperature have been known to change every few hours, let alone the wind, and it's my opinion these factors have a greater bearing on your potential shot than does elevation alone.
Only when time constraints force it do I depend on a 100 yard zero and ballistic programs. I believe everyone should shoot at two and three hundred yards, if for no other reason than to better appreciate wind drift.

Stubblejumper: Yes altitude has a noticeable effect on a bullets trajectory!
I second Mr. Johnson's advice to you regarding running some trajectory tables online.
Long before the home computers sophisticated ability to figure this one out - I proved this fact to myself!
Example (exhibit) #1: I own and have used a wonderfully accurate custom Rifle in caliber 240 Weatherby Magnum (YES Weatherby cartridges are capable of wonderful accuracy!).
Each year I would sight in verify my wonderfully accurate Rifle, at my range located on the tide flats of the Duwamish (Green) River in Seattle, Washington! This range is located at about 20' elevation! I always had it set for 2.5" high at 100 yards there at sea level!
Many times, once I travelled to Antelope country (and in Wyoming this is often at 6,000' plus) I would take my Rifle and verify the sight in. This I felt neccessary to do once I had travelled 1/20th the way around the earth with the worries that entails!
That Rifle often then shot 3.5" high at 100 yards (sometimes more!)!
Then, I would use it on my Hunt and head home. A couple of times I would run right back out to my "sea level" range just to check and make sure the Rifle did not have a case of the "wandering P.O.I's (point of impacts).
It didn't - that wonderful Rifle holds its point of aim from year to year like clockwork!
It was the change in altitude that changed my P.O.I.!
Yes the changes in altitude DO affect a bullets flight and sometimes noticably for the Hunter.
I have heard that the atmospere is only 85% as dense at 6,000' altitude as it is at sea level - I don't know this for fact I just heard it!
I think I will go to my NOW available online trajectory (ballistics) site and enter some numbers.
I now live at 5,400' altitude and often Hunt up to 8,000'! And when Hunting Antelope I often do that at 2,300' elevations here in eastern Montana!
Hold into the wind
VarmintGuy

IIRC it also tends to be rather humid around Seattle. . Humidity is another one of the factors that contributes to air density. So in other words going from a humid sea level to a dry high alititude location you will normally see more altitude effect than if your original sight in location was in a dryer local.
Your MOA difference isn't surprising at all!............DJ