Question about high altitude shooting

[6mm06guy]

I just got back from Colorado where I killed a 4x4 bull at 647 yards. My question pertains to my rifle. I'm using an 8mm rem mag with 200gr accubonds at 3000 fps. Whats weird, probably because I have never shot at a high altitude is I had to put my cross hair right on the animal to score a hit. zero drop at 647 yards. It boggles my mind. I drove home to Missouri and double checked my zero, still dead on. 1" high at 100 yards. The accubonds bc is .450, even at altitude it should have had like 30+ inches of drop, and if not I need a new ballistic calculator. Or would my zero be higher at 100 at elevation?

any advice?

 

[HARPERC]

Always zero at hunt location when possible.

 

[dok7mm]

Wondering how you knew to hold on the bull?

 

[6mm06guy]

Wondering how you knew to hold on the bull?

I missed 6 times until I seen a ground strike and the bull didn't move, just luck.

 

[dok7mm]

Bet that was pretty nerve wracking.

 

[6mm06guy]

Very, but I got the job done cleanly.

Bet that was pretty nerve wracking.

 

[ShtrRdy]

Were you shooting uphill or downhill? That will make your bulle hit higher too.

 

[6mm06guy]

Were you shooting uphill or downhill? That will make your bulle hit higher too.

Slightly up hill

 

[elkaholic]

Altitude definitely makes a difference but you would have to be in outer space to shoot dead on at that range with a 100 yard zero; or an extremely steep slope as stated already!

 

[6mm06guy]

Altitude definitely makes a difference but you would have to be in outer space to shoot dead on at that range with a 100 yard zero; or an extremely steep slope as stated already!

I'm not sure exactly how it all worked, I zeroed in Missouri at 100 1" high (800' altitude) went hunting and had to put it right on him at 647 yards to get a hit at 8600'. Wierdest thing I have ever had happen. the only thing I can think of is the gun must shoot high at 100 at 8600'.
I double checked my zero back in Missouri,( perfect) and killed a doe an hour ago here in Missouri

 

[elkaholic]

I did the math on JBM and the difference is 10.1" going from 800 to 8500', all else being equal......Rich

 

[6mm06guy]

I did the math on JBM and the difference is 10.1" going from 800 to 8500', all else being equal......Rich

That makes alot more sense. Thanks. I just didn't think there would be that much difference

 

[Barrelnut]

Assuming your 1" high sight-in at 100 yards givers you a 200 yard zero...

There would have been about 84" drop from a 200 yard zero at 800 ft. elevation and a 74" drop from a 200 yards zero at 8600 ft. elevation. So the rifle would have shot a 10" flatter at altitude in CO. A elk is about 24" from back to bottom of chest. So, there is some leeway. With a MOA rifle and depending on where the reticle was when the hammer dropped, the 10" difference caused by the elevation change might not have mattered.

That is a physical drop of 6 ft. at 647 yards though. Probably why you finally saw one hit the dirt low and were able to compensate. Maybe, with the excitement, you just don't remember compensating on that last shot.

 

[6mm06guy]

Assuming your 1" high sight-in at 100 yards givers you a 200 yard zero...

There would have been about 84" drop from a 200 yard zero at 800 ft. elevation and a 74" drop from a 200 yards zero at 8600 ft. elevation. So the rifle would have shot a 10" flatter at altitude in CO. A elk is about 24" from back to bottom of chest. So, there is some leeway. With a MOA rifle and depending on where the reticle was when the hammer dropped, the 10" difference caused by the elevation change might not have mattered.

That is a physical drop of 6 ft. at 647 yards though. Probably why you finally saw one hit the dirt low and were able to compensate. Maybe, with the excitement, you just don't remember compensating on that last shot.

Except I missed the first 6 shots and hammered him twice in the last 2 shots.i believe that almost all of you all right and it doesn't make any sense to me either it just seems like that's how it went