Thread: What's the Deal?

I did some shooting the other weekend with my .260 Rem and 140gr A-Max combo. From 9 measured shots through the chrony I got an average of 2644fps. Using Litz measured G7 BC of .299 for the A-Max Leathel entered all the necessary digits in to his Shooter app for 500m and I think it said 3.2 mils up. I shot it at that and I saw I was about a mil low. I adjusted up .8 mil to 4 mil, shot it again and it was bang on. We had a play with the input speed and to get that calculation it had to be set at 2500fps. WTF? We moved back to 650m and input everything the same as before but adjusted the speed for 2500fps, it said 6.2 mil up. Adjusted and shot it, and it was bang on. I have since entered this into JBM as well and got the same results with the muzzle velocity set at 2500fps. Any ideas what might be going on?

Thanks

Was atmospheric conditions included?

Originally Posted by Wirehunt

Was atmospheric conditions included?

That would be my guess too. Either incorrect atmospherics in the calculator, or your chrony is out. Can't think of anything else that would have a repeatable effect like that.

Originally Posted by StrikerNZ

That would be my guess too. Either incorrect atmospherics in the calculator, or your chrony is out. Can't think of anything else that would have a repeatable effect like that.

+1

Were there angles involved ?

Did you input the baro pressure or altitude ?

How "good" is your crony ?

I'm not sure what baro was I think we just guessed? Leathel might remember. Angle was corrected for also at 5 degrees. Chrony used was a master chrony. We were getting pretty repeatable results with it but not 100% sure on it's accuracy.

Could Baro cause me to be a whole mil out at 500m? Speed should be close. If anything I'd expect it to be faster than what the chrony was reading but I seem to have a slowish barrel (tight bore).

Ive got a master chrony and its ok. But you need to verify your drop regardless. It could be alttitude atmospheric settings also. Not being a wise guy here but you didnt mix n match with yards and meters did you? Ive seen that before too.

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Nope. Was definitely meters and I got the same result from home on JBM.

Unless the worlds about to end I don't think atmo will make .8 mill difference at 500. Scope height- accurate zero off the same platform that's being shot etc will all make a diffference . but its far more likely an error from a chrono that relys on ambient light.

Originally Posted by sneeze

Unless the worlds about to end I don't think atmo will make .8 mill difference at 500. Scope height- accurate zero off the same platform that's being shot etc will all make a diffference . but its far more likely an error from a chrono that relys on ambient light.

+1 Chrono, zero or your surface your shooting off has changed if using a pod.

I re-zeroed minutes before the long shots.

Originally Posted by Normie

I re-zeroed minutes before the long shots.

That leaves 2 other things to eliminate, or a data imput that is causing the difference. What is the book speed for your load? Light for the chrony could be a factor as well.

Do you have a load zero for powder temp. 20degrees can fuck your shit all up.

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Not sure what the book speed is but I'd imagine I'm slower than it should be. The day was perfect for the chrony too. Nice clear day 24 degrees or so in the middle of a paddock.

The powder used is ADI 2209 so fairly temp stable.

If it helps my load is
Lapua .260 Brass
140gr A-Max
42gr AR2209
CCI BR primer

22" MAB 1:8 barrel

Originally Posted by Normie

Not sure what the book speed is but I'd imagine I'm slower than it should be. The day was perfect for the chrony too. Nice clear day 24 degrees or so in the middle of a paddock.

The powder used is ADI 2209 so fairly temp stable.

If it helps my load is
Lapua .260 Brass
140gr A-Max
42gr AR2209
CCI BR primer

22" MAB 1:8 barrel

There are many variables I know but I used 46grs 2209 in my .260AI and only got 2820fps thru a 24" barrel with the A max. For the same charge in RL-17 I get 2920ish with the same projectile.

Wow. That's a lot slower than I would have thought. I did a ladder for Hodgdon Hybrid 100V and got much better results. At 44gr with no pressure signs at all I got high 2700s fps. 150fps faster for 2 grains more. So I will get some more and do a full work up.