Originally Posted by Boss Hoss
Also—when looking at trajectory at 1k take your high and low velocity and look at the vertical dispersion because it will open your eyes to see how important that is. The other point is when testing at long range or short NEVER chamber (just start it in the chamber not fed from the magazine) your round until the sight picture is perfect and the condition is right. Reason is that in a warm chamber will “preheat” your round which is a pressure variable that you do not want in load development or in competition.
I've tried to get my long range shots off between 10 and 15 seconds of chambering them. If they cook in the chamber longer than 20 to 30 seconds, the hotter powder shoots bullets out faster and they go high. So if the wind's changing and I need to keep up with it on the sights, I'll come down 1/4 MOA for every 30 seconds the round's cooked in the chamber. Works just fine. I've waited as long as 3 minutes and after coming up 5 or 6 clicks, put the next bullet down the middle. Then come back down 4 or 5 clicks and the hotter barrel's now back to shooting about normal for the next shot.
Conversely, there's been folks at the High Power Nationals with a match winning record setting score almost finished on the 1000 yard target and a cease fire was called 'cause of boats in the impact area on Lake Erie. If the wait before continuing to fire is longer than a couple minutes, competitors get one sighting shot before shooting the next record shot. A few have got the OK to continue after about a minute and a half wait and their barrel's cooled down enough that the next record shot didn't heat up enough in the chamber to shoot its bullet out as fast as the previous ones. That low velocity shot ends up in the 9 ring at 6-o'clock and they lose the match.