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Velocity Calibration
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<blockquote data-quote="brant89" data-source="post: 1207076" data-attributes="member: 49553"><p>I recently decided to switch my 260AI over from a steady diet of 140gr AMAX's to 140 BTHP's since the AMAX is discontinued and the BTHP is cheap. After two ladder tests I found a load that should be quite accurate out to 1000yds (though velocity was surprisingly low) and went out to confirm my velocity. I verified my 200yd zero and proceeded to move back in increments out to 1000yds. At 300 and 520yds, my ballistic calculator agreed that my velocity was 2650fps (I realize this is quite slow for a 260AI, since it is only used for target practice it is a non-issue for me since accuracy is quite good). Then I backed up to 700yds and I was hitting over a minute high, but still grouping very well. So I used that velocity to calculate my 980yd solution and I impacted about one minute high again. I saw this same phenomenon occur before with the AMAX, its like my BC is INCREASING after about 600yds (which is obviously not the case). I played around with the velocity, BC, and elevation correction (thinking the scope may not actually be 1/4 MOA clicks) but nothing produced a solution that matched my results. Conditions were 75 degrees, sunny, 8-12mph winds shooting across a bare dirt field with rolling hills. I can only think of two possible causes; either my scope (Vortex Viper PST 6-24x50) has a mechanical issue where I'm losing about 1MOA as I pass 0 on my turret, or I have a lot to learn about shooting in heavy mirage. Any thoughts on this?</p></blockquote><p></p>
[QUOTE="brant89, post: 1207076, member: 49553"] I recently decided to switch my 260AI over from a steady diet of 140gr AMAX's to 140 BTHP's since the AMAX is discontinued and the BTHP is cheap. After two ladder tests I found a load that should be quite accurate out to 1000yds (though velocity was surprisingly low) and went out to confirm my velocity. I verified my 200yd zero and proceeded to move back in increments out to 1000yds. At 300 and 520yds, my ballistic calculator agreed that my velocity was 2650fps (I realize this is quite slow for a 260AI, since it is only used for target practice it is a non-issue for me since accuracy is quite good). Then I backed up to 700yds and I was hitting over a minute high, but still grouping very well. So I used that velocity to calculate my 980yd solution and I impacted about one minute high again. I saw this same phenomenon occur before with the AMAX, its like my BC is INCREASING after about 600yds (which is obviously not the case). I played around with the velocity, BC, and elevation correction (thinking the scope may not actually be 1/4 MOA clicks) but nothing produced a solution that matched my results. Conditions were 75 degrees, sunny, 8-12mph winds shooting across a bare dirt field with rolling hills. I can only think of two possible causes; either my scope (Vortex Viper PST 6-24x50) has a mechanical issue where I'm losing about 1MOA as I pass 0 on my turret, or I have a lot to learn about shooting in heavy mirage. Any thoughts on this? [/QUOTE]
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