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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

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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... :-)

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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?

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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?

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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

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