Zeroing tips needed.

[shred]

Barometric pressure won't change PF at the chrono either.  There's just not much air to get through before the chrono screens.

 

The variables to care about are temperature, shot-to-shot variation and chrono differences.  Everything else is in the noise.  Load so you're safe on all of these and never sweat a chrono stage again.

 

 

 

[SGT_Schultz]

6 hours ago, waktasz said:

So when it's 95 degrees you'll be at 124pf. Good to know

 

I've never seen a smokeless powder that exhibits a negative temperature coefficient.  They get more energetic the hotter their temperature, not the other way around.

 

His two PFs are so close that they likely fall within statistical noise + instrumentation error.

[waktasz]

WST is reverse temp sensitive I believe 

[zzt]

I'm going to weigh in on chrono techniques, and probably several other things.  When you chrono, the bullet's path MUST be down the long axis of the chrono and parallel to the viewing ports.  Anything else reports lower velocities.  There is a problem with Open guns in that the dot is so high over the bore.  The chrono ROs are told to aim at the same spot every time, no matter what.  So if you chrono'd properly and got 170PF, you may chrono a little less at an LII or LIII chrono station.

 

When I shot 40sw Open my ammo was 172 PF, because the SDs were not as good as I liked.  With 9mm major my loads are 169PF.  The slowest in a 20 shot string is 168PF.  So I'm safe unless both chronos at the match are way out of whack.  Supposedly that can't happen because they use special ammo to calibrate them.  I'm not 100% sure that happens at every match.

 

Regarding bullet drop at ranges other than your zero range.  Use on of the good ballistic calculators available online.  You know your velocity at 10 yards.  So add 20 or 25 and plug that into this calculator.  http://gundata.org/ballistic-calculator/  Then play what if on zero ranges to your hearts content.

 

 

[zzt]

3 minutes ago, waktasz said:

WST is reverse temp sensitive I believe 

 

Actually, it is weird.  It is most energetic at mid-range temps, and gets slower at hotter AND cooler temps.  If you are one of the daring types that likes to stay close to the minimum PF for your Division, make sure you chrono when it is really cold, or really hot.  That way you'll be sure to make PF anytime.

[waktasz]

Special ammo and calibrating the chrono? ha

[shred]

7625 reliably generated faster velocities when cold in Open 38SC loads.  Got lots of chrono data on it. 

This series was the same gun, same ammo batch, same day, same range, same chrono (Oehler 35P).  10 shots per average.

 

125 HAP 7.7gr 7625 1.240 OAL.  New Starline SC brass.

 

1342 PF 167.8   Warm (90+'F)

1363 PF 170.4   Room Temp (AC indoors)

1372 PF 171.6   Ammo from fridge

1387 PF 173.4   Ammo from freezer

 

I don't understand the chemistry of it, but it's well documented in some powders. 

 

As for special chrono ammo, it's the popper calibration ammo.  All it needs to do is be between 115 and 125 PF and chrono the same from day to day +/- 4% (which is about 5PF!) in single-chrono setups. So if the chrono is consistently wrong, the calibration ammo won't catch it.