I planned on sighting in my rifle to different ranges and recording the MOA and using that in CO. Where we are going in Maybell, it is 7000ft elevation. The "guide" told me to sight my rifle in at 100 here in FL, and it would be dead on at 300 yards. Is this guy crazy because I didnt know elevation could be this critical.
Depending on what your bullet/velocity combo is, only when you get as far out as 400 yards or so will this much elevation change mean that point of impact changes more than a couple inches from at sea level.
So, make your own in-the-field drop table at home and rely on it for the hunt out as far as 400.
Do you have a ballistic program to use to compare the two at different altitudes?
The rifle will not be zeroed at 300 yards, that is a certainty, and you will not even be off your Florida 100yd zero by more than .04 MOA as well, ya .040". The guy obviously doesn't know ballistics very well so consider the source when he offers other advice concerning it too. [img]images/icons/wink.gif[/img] Use what you know and have proven to yourself before the hunt.
What will be most important at LR is how stable your MV will be in the colder temps up there. A slower MV and a lower air density would in effect cancel some of the others effects out, but by how much you'd not know unless it was checked. What I would do: Check MV over a chrono when you get there if you can. If you can't, shoot the load at 300 yards, 700yds and 1000yds and record the come ups in MOA needed. You should already be certain of the BC the bullet has, the MV, and verified drops before heading off on the trip. Print out drop charts for the altitude (7000) and temps you're likely to shoot in. This is where a Kestrel and a Palm Pilot would be sweet. When you shoot there, the drop chart that indicates the drop you're getting will likely be one at a colder temperature if the loads MV is slower up there, both an air density increase from the cold and/or slower MV will result in more bullet drop. If you print some drops out at various temps with/and various slightly slower MV's you'll find a good combo in one that matches the actual drops you record when firing them there.
Like Len said, the effect of the "100 yard zero changing because of altidude" (air density) is minimal, even at 1000 yards it only amounts to what, less than 1/2" high on the target. But, trajectory changing from what you are used to will mean about 2.5" high at 500yds, 9" high at 700 and, 32" high at 1000 yards... This is what will really kill you, not to mention an error a MV change could cause. But like I said, the MV error would surely be in the opposite direction as the air density error is, and if you correctly compensated for the air density change and didn't figure on what might be a lower, maybe much lower MV, you would without a doubt be then hitting lower than POA.
Good luck on the hunt! Sorry for all the suggestions you probably didn't even need on this, answering questions you didn't even ask. [img]images/icons/grin.gif[/img] [img]images/icons/smile.gif[/img]
Smoak, take heed of what Brent said. The change in MV due to temp change is more important then air density, humidity, and altitude.
I would suggest you develop your load using an insensitive powder like Hodgdon Extreme. These powders are also very accurate and clean burning. I develop my loads at various times of the year and the vel/drops are within the error of the day from 35C to -10C. This is a big confidence builder to know that you ammo works regardless of what the outside temp is.
The barrel is another story. Some do change POI when very cold. So the best advice I can give you is get there early. Let your gear equilibrate to the local temp/humidity. Shoot in the conditions you will hunt in to verify your sight in and drop.
You should sight your gun in anyways incase it moved during the trip. Shoot from a cold barrel to confirm LR drops and voila, you are good to go.
During this sight in and drop test, you may want to have your guide watch so that he understands that you know what you are doing at longer ranges. You don't need to have a debate about whether you can or can't hit your record animal when it steps out at distance.
Just looking at this chart I had been comparing altitudes on, a small MV reduction of 30 fps would mean an additional drop of 2.5" at 700yds and 5" at 1000yds.
A 50 fps reduction in MV would mean almost 4" at 700yds and 9" at 1000yds.
Some more food for thought:
Looking at reductions in MV and how well they compensate for a lower BP at 7000 ft. using standard conditions at altitude, with the round shooting 9" flatter at 700yds and 32" at 1000yds as refered to in my example before this;
If the MV was reduced by 165 fps by a temp sensitive powder being used, at 700 yds the bullet would be about 4" high and at 1000yds it would be zero, or right on POA.
If the MV was reduced by only 120 fps, the bullet would be zero at 700yds and still 21" high at 1000 yards though.
Point is, a slower MV will not change the change the trajectory curve in the same way air density will. A change in BC from the change in air density flattens the curve, dropping the MV steepens the trajectory curve and one will never balance the other out throught the whole curve.
If using a temp insensitive powder already, you could make charts using a higher BC bullet in Florida to simulate the lower drag you'll be shooting in at altitude, but you're probably shooting a pretty high BC bullet already.
If you were planning on hunting with a 178gr A-Max for instance with a .5 BC, that BC adjusted for 7000 ft. ASL would require a .610 BC to shoot at sea level in Florida to get drops for real close charts to use on the hunt.
I have some altitude, temp charts, for the 175 Gr. Blackhills MatchKing at 2600fps. Perhaps I might join the fray.
Taking a standard sea level zero of Altitude 0 Ft. Temperature 59 F and Humidity 78% with sights on at 100 Yards. At sea level the bullet will be 5MOA low at 300 yards.
Without adjusting the scope turrets, the effect due to altitude at 7000ft will place the bullet 0.3MOA higher at 300yards than at sea level.
Now factor in the change in air density due to the approx 23 F drop in temperature between altitudes. Droping air temperature makes air more dense. Factor in additional drop of approx 0.06 MOA at 300 yards due to temperature.
To sum up
100 Yard drop at sea level = 0 MOA
300 Yard drop at sea level = -5 MOA
300 Yard press adjust. for 7000ft alt. = +0.3 MOA
300 Yard temp. adjust. for 7000ft alt. = -0.06 MOA
Summing the parts gives bullet placement at at 7000ft at 300 yards as approx 4.76 MOA below your sealevel zero. Thats 14 Inches. The bullet with plot 0.24 MOA higher than a 300 yard shot at sea level. Thats 0.72 Inches. Peanuts.
Lets look at whats happening at 100 Yards
100 Yard drop at sea level = 0 MOA
100 Yard press. adjust. for 7000ft alt. = +0.28MOA
100 Yard temp. adjust. for 7000ft alt. = -0.006 MOA
Summing the parts gives bullet placement at at 7000ft at 100 yards as approx 0.274 MOA above your sealevel zero. Thats quater of an inch.
All calculations are approx. Reductions in MV due to temperature have not been considered.
The important thing to remember is that the altitude change will make have no pratical affect on your sea level drop chart out to 300 Yards. Things go ballistic after 300 yards, and at 1000 yards, the Black Hills cartridge could be hitting 50 or more inchs above you sealevel tradjectory.
As you will be hunting at high altitude the possibility of an inclined shot is quite high. A 40 Degree inclination will put this load 9 inches high at 300 yards. Inclination is where you should concentrate your efforts.
Apologies for the long posting, especially as I am going over ground already covered by Len Backus,Brent and Jerry Teo.
It's really splitting hairs but, using your bullet and speed I come up with something a bit different.
175gr SMK at .505 BC at 2700 fps.
Where you get: 100 Yard press. adjust. for 7000ft alt. = +0.28MOA
At 100yds I get .02" less drop at 7000ft ASL(2.49" total drop to 2.47"), not +.28". From a BP of 29.53in HG to a BP of 22.75in HG or a BC that goes from .505 to .6565.
A +.28" POI seems out of place, doesn't it?
At 0 ft. ASL and at 100yds I get .01" more drop from a temp change of 59deg F to 34deg f, or a BC that goes from .505 to .4792
A combined change from 0 ft./ 59deg F/ 29.53in HG to 7000 ft./ 34deg F/ 22.75in HG resulted in a drop from:
100yds = 2.49" to 2.47".
1000yds = 411.09" to 362.64"
A change of .1174 BC (.505 BC to .6224 BC)
A drop of 25deg alone at 0 ft. ASL results in a change of drop from;
100yds = 2.49" to 2.50"
1000yds = 411.09" to 423.30
a change of .0258 in BC. (.505 BC to .4792 BC)
At 7000 ft. ASL the same temp drop results in a change of drop from;
100yds = 2.47" to 2.47"
1000yds = 355.21" to 362.64"
a change of .0341 in BC. (.6565 BC to .6224 BC)
At 0 ft. ASL and 59deg F. a change in BP from 29.53 to 22.75 would result in a change of drop from;
100yds = 2.49" to 2.47"
1000yds = 411.09" to 364.63"
a change of .1169 BC (.505 BC to .6219 BC)
At 0 ft. ASL and 34deg F. a change in BP from 29.53 to 22.57 would result in a change of drop from;
100yds = 2.50" to 2.48"
1000yds = 423.30" to 371.02"
a change of .1159 BC (.4792 BC to .5951 BC)
Food for thought. [img]images/icons/smile.gif[/img]
The real pisser is these drags are based on a G1 drag curve, not the G5 which better models this bullets form, a custom drag model with the 175 SMK's exact form as the basis would be even better. [img]images/icons/wink.gif[/img]