altitude change question

D

DirkTejan

Guest
Hello again!

For those of you that were so kind as to help me to determine my previous problem - it turned out that the scope rings were not quite tight. Thank you all much for your attempts to beat some sense into me.

Now on to my next dilemma….

I have in my notes some tips from one Jerry Hensler, who appears to a Benchrest shooter of some repute. Mr Hensler suggests that to keep your rifle shooting little groups, your powder charge should increase as air pressure drops. If for example your charge was 29.5 grains (6PPC) at 2700 ft altitude, you should use a charge of 29.9 grains at an altitude of 6400 ft. As I am shooting a 6XC with a 107 SMK, my charge at 2700 ft altitude is 36.0 grains, so I would imagine a charge of 36.5 grains would work at 6600 ft where I expect to be shooting.

So assuming that is correct – I have a question…..

If I am currently getting 2985 fps at 2700 ft altitude using 36.0 grains- what would you suppose my velocity would be at 6600 fps using a 36.5 grain charge?

Thanks-
Dirk
 
Ah yes...

Powder is H-4350. Right now here (Las Vegas) (2700 ft) it is around 90 and where I am going (Price UT) (6600 ft) it will probably be about 75 degrees F.

Actually - the load was tested for accuracy at 70 degrees F at 2700 ft altitude.

Thanks-

Dirk
 
This is a sketchy way of going about it without testing but...

At 2700 Alt. and 70 degrees and 2985 fps my ballistic computer says you need 24.75 moa at 1000 yards.

When I change the Altitude to 6600 and the temp to 75 degrees you need 22.75 moa at 1000 yards at 2985 fps.

When I leave the Alt at 6600 and the temp at 75 degrees I have to alter the bullet speed down to 2880 fps to equal the 24.75 moa at 1000.

In short, you need to loose 105 fps for that one distance.

Now here comes the part I cannot do without testing. In one of my magnum's data books I recorded as much as 7 fps per tenth of a grain for light loads H4350. When I look at the hotter loads I was only gaining 3 fps per tenth of a grain. In theory you need to drop as little as 1.5 grains and maybe as much as 3.5 but don't you dare do any of that if it is outside of what you have tested or the Min./Max in a load book.

I never really thought of doing it this way and it was fun on paper. In real life all you need to do if use a ballistic computer and that little dial on the scope to make the proper correction without touching your charge.

Keep your charge and crank that knob! :D
 
thanks Tony

The reason I am asking......I plan on shooting a precision rifle match (tactical- whatever) and I want the very best accuracy (of course) but at the same time - they normally hit you right off the bat with a cold bore shot - no sighter allowed- usually at long range.

Thanks for the help.

Dirk
 
I shoot Tactical Matches too and my advice is still the same. I use my ballistic computer to make a "data card" before I go to the match and I usually crank in my offset for elevation & temp. before I leave home or sometimes in the hotel the night before.

I never change my loads for those matches. I do shoot moly bullets which in my case help with the "cold bore" and I also pre-foul my barrel before I leave home. You might want to consider those last two but the data card it a must.

Good luck with your match.
 
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