OzFclass

We shoot around central Victoria where the average height above sea level is about 750 ft. When we go to Canberra which is 2000 ft above SL we use the same load. However going down to SL is a different proposition but it depends upon your MV
.Going to SL from where we shoot would cost us about 50 f/s in terminal speed --no problem for 900 yards but is definitely a problem for 1000 yds IF your MV is too low to start with.

That is why I recommend a MV of 3000 to 3020 f/s --it covers all contingencies.
For other calibres with higher BC's this may not be so critical.
My honest opinion ( and it's just an opinion ) is that 1000 yards is really stretching the limit of the 155 gn projectile because variations in terrain and atmospheric conditions have a marked effect on it at extreme ranges --and this forces you to take loads to maximum and beyond, or have ridiculously long barrels.
And this applies to the heavier projectile as well. You may well have better BC's but you cannot run them as fast, so the gain is marginal, and probably non existent for a good wind reader.
Just an opinion.

Denis it would be a good idea to put a few warm-up shots through the barrel to get it to operating temperature before you take chrono readings.

David, when Barry was a much younger man he used to travel to Williamstown Victoria for teams championships and Queens Prize meetings at sea level with a hotter load as he has indicated. I never forgot that as a young bloke then.

What matters is the air density and how that effects velocity and moves you away from your tuned velocity. Of course temperature is the biggest influence on air density and the elasticity of the air mass and humidity to much lesser degree once you have set up at your local range. But if you move to other venues with significant elevation difference then the air pressure differences measured by barometric pressure becomes significant. Such that traveling between Australia and elevated venues in the states you have to use a different twist rate to stabilize the bullet.

Pilots consider a measure called, ‘Density Altitude’ when calculating fuel needs and many racing car drivers use the measurement to adjust the efficiency of their carbies at different venues or computerized variation of that. Interesting that there is a link between oxygen levels and ignition efficiency too.

Shooters been using the hand held devices and make adjustments to tuners based on increments of 500 ft density altitude changes. This is fairly close, but variable because at different elevations the atmospheric gas mix alters e.g. more oxygen changes both the elasticity of the atmosphere and effects bullet drag; and like the hot rodders, the efficiency of the ignition is changed too. If you use a density altitude meter, such as a Kestrel, in significantly different elevation settings your tune will be affected by a) drag, b) appropriate twist rate and c) ignition efficiency. So your meter readings won't necessarily correspond to tune with density altitude changes.

In short, you need to base your tune at the competition venue and get a hometown advantage. The other good strategy is as Barry has indicated, use a faster load because in percentage terms the disadvantage is less and because a full case is a more efficient burn too so long as your ammo is kept out of the sun.

Just saw this bulletin from accurate shooter and had a giggle about the timing v's this thread.

http://bulletin.accurateshooter.com/201 ... mperature/

Very timely Denis. You guys have to act on it and graph the temperature range. Do it once and its pretty transferable to other rifles with the same setup. The old Silhouette ballistics program allowed you to input the observations into the trajectory. Maybe Quickload could do similar things once you have a base of actual observations. You will never get Alzheimer's being a target shooter. David.