hunter686 Posted February 24, 2016 Report Share Posted February 24, 2016 Anyone have any experience with this? I had my 204 doped superbly after about 50 rounds and a day messing around with targets last summer. As of a couple of weeks ago I was noticing my shots were coming in consistently low by about 1 moa. The zero was still bang on at 100 but I ended up having to tweak my numbers to account for it distances after that: Previous: 2730 fps BC:260 (287) Now: 2600 fps BC: .25 (287) I understand there will be atmospheric influences and I have accounted for these as best I can. The only major one would be the temperature from 15° to now about 5-10°. Anyone have any reason why my trajectory has shifted so much? Cheers Hunter686 Link to comment Share on other sites More sharing options...
gbal Posted February 24, 2016 Report Share Posted February 24, 2016 Hmmm....temperature! Sierra tested various hunting cartridges and found something like 2.5 to 4 fps drop per C degree ... their 30-06 went from2700-2600 with a rarher large for uk 50 f degree drop (sight in summer,hunt winter) with drops of 1" at 200,2" at 300,4 at 400y below the summer zeroed drops. Other estimates are around 2.5-4 fps per C degree(or 1.8F degree)-again dependent on cartridge velocity,and actual temperatures....maybe more effect on the zippy fast items,but no good data on that yet...Fclass etc will have some,but for much heavier bullets/calibers.** So in your case,you might have 5 to 10 C degree colder.....which might be 40fps -maybe more in the 204 (almost all data is on hunting cartridges-as above-that's the likely scenario,and it may matter-no buck for your bang!) So very ballpark,some drop is certain-exactly how much......(are your velocities -and 100fps drop-estimated from the actual drop data,rather than chrono?). But drop in temperature will be contributing..... gbal ** for F**ing cold shooting,the 20 or 22 Brrrrrrr... might be worth a look... :-) Link to comment Share on other sites More sharing options...
BlueBoy69 Posted February 24, 2016 Report Share Posted February 24, 2016 Hi, what were your loads, as a figure of 2,760 and 2,600 fps seems very low for a 204, even with the heavy 55 gr Bergers? Going by your rifle, a Tikka T3 SS, that has a 600 mm barrel, the 32 gr bullets should be moving about 3,750 fps, and a 55 gr about 3,000 fps. And as gbal mentioned, how did you get the velocity. Were they over a chono, or estimations? Link to comment Share on other sites More sharing options...
hunter686 Posted February 25, 2016 Author Report Share Posted February 25, 2016 Hmmm....temperature! Sierra tested various hunting cartridges and found something like 2.5 to 4 fps drop per C degree ... their 30-06 went from2700-2600 with a rarher large for uk 50 f degree drop (sight in summer,hunt winter) with drops of 1" at 200,2" at 300,4 at 400y below the summer zeroed drops. Other estimates are around 2.5-4 fps per C degree(or 1.8F degree)-again dependent on cartridge velocity,and actual temperatures....maybe more effect on the zippy fast items,but no good data on that yet...Fclass etc will have some,but for much heavier bullets/calibers.** So in your case,you might have 5 to 10 C degree colder.....which might be 40fps -maybe more in the 204 (almost all data is on hunting cartridges-as above-that's the likely scenario,and it may matter-no buck for your bang!) So very ballpark,some drop is certain-exactly how much......(are your velocities -and 100fps drop-estimated from the actual drop data,rather than chrono?). But drop in temperature will be contributing..... gbal ** for F**ing cold shooting,the 20 or 22 Brrrrrrr... might be worth a look... :-) Interesting! The velocity is based with a set, real time BC of the SBK 39's of .260 (.287) then I just tweaked the velocity to accommodate the drop. I have accounted for the temp in my ballistics app. Is there maby a setting or something I may have moved that has caused the shift inadvertently? Link to comment Share on other sites More sharing options...
hunter686 Posted February 25, 2016 Author Report Share Posted February 25, 2016 Hi, what were your loads, as a figure of 2,760 and 2,600 fps seems very low for a 204, even with the heavy 55 gr Bergers? Going by your rifle, a Tikka T3 SS, that has a 600 mm barrel, the 32 gr bullets should be moving about 3,750 fps, and a 55 gr about 3,000 fps. And as gbal mentioned, how did you get the velocity. Were they over a chono, or estimations? My mistake **3730 **3600 Too busy looking at 7mm charts Link to comment Share on other sites More sharing options...
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