Chrono Question

[22~308TargetShootr]

How much difference in MV can a couple feet difference in distance from muzzle to Chrono make ? Shot my 155's a week earlier and was getting averages 2950 +, was first time used chrono and didn't measure or take note of the distance, althouogh I know it was right in the area of 10 feet or maybe closer, but not as close as 5. Shot again just Tuesday and shot the same load, nothing changed , (seating depth, charge, etc.) this time though I measured out exactly 10 feet from muzzle, and never reached 2950 once, averaged right in the area of 29teens - 2920's. I know day conditions can have an effect but it was also essentially same temperature and conditions between the two days. Could I lose on average 40-50 fps with just a few feet difference in distance to chrono ?

 

[armorpl8chikn]

Re: Chrono Question

You can lose 40-50fps just from a cloud passing overhead. The only way to get an accurate chronograph is to spend 15K on Doppler. Now you turds flame away.

 

[MALLARD]

Re: Chrono Question

no , check jbm ballisitics , you'll see you'll see a 10fps difference between 5y and 10y. 40 fps doesnt make sense. Even a 45 or 9mm wouldn't loose that much velocity between such a small distance.

probably the weather or perhaps variations in your loads

Trajectory
Input Data
Manufacturer: Sierra Description: MatchKing™ (Litz)
Caliber: 0.308 in Weight: 175.0 gr

Muzzle Velocity: 2650.0 ft/s Distance to Chronograph: 15.0 ft

Calculated Table
Range Drop Drop Windage Windage Velocity Mach Energy Time Lead Lead
(yd) (in) (MOA) (in) (MOA) (ft/s) (none) (ft•lbs) (s) (in) (MOA)
0 -2.0 *** 0.0 *** 2659.3 2.357 2747.5 0.000 0.0 ***
1 -1.9 -181.0 0.0 0.0 2657.4 2.355 2743.6 0.001 0.2 19.0
2 -1.8 -85.5 0.0 0.0 2655.6 2.354 2739.8 0.002 0.4 19.0
3 -1.7 -53.7 0.0 0.0 2653.7 2.352 2736.0 0.003 0.6 19.0
4 -1.6 -37.8 0.0 0.0 2651.9 2.350 2732.1 0.005 0.8 19.0
5 -1.5 -28.3 0.0 0.0 2650.0 2.349 2728.3 0.006 1.0 19.0
6 -1.4 -21.9 0.0 0.0 2648.1 2.347 2724.5 0.007 1.2 19.0
7 -1.3 -17.4 0.0 0.0 2646.3 2.346 2720.7 0.008 1.4 19.0
8 -1.2 -14.0 0.0 0.0 2644.4 2.344 2716.9 0.009 1.6 19.0
9 -1.1 -11.4 0.0 0.0 2642.6 2.342 2713.1 0.010 1.8 19.0
10 -1.0 -9.3 0.0 0.0 2640.7 2.341 2709.3 0.011 2.0 19.0

 

[Rthur]

Re: Chrono Question

I guess my first question would be, what chrono are you using? I would also take a tape measure to try to keep your testing as consistent as possible. Setting up to close can give false speed measurements from muzzle blast.

 

[Deadshot2]

Re: Chrono Question

Using an "Instrument Correction Calculator included in my ballistic's program, the difference in speed would be about 4 fps than if the chrono was at 10 feet from the muzzle.

Biggest factor for uniform variation in speed is temperature. For irregular variations look to neck tension variations as well as a chronograph that isn't supported on a steady mount. If it's wig-wagging from the muzzle blast it's not going to be consistent in its readings.

 

[EMorr]

Re: Chrono Question

As others have said there could be a few different reasons for seeing different readings on different days but I’m in agreement with <span style="font-weight: bold">armorpl8chikn</span> as to the reason you are seeing a difference. What were your ES and SD numbers for the load on the different days? If the ES and SD number were near identical on both days I would say a cloud caused the discrepancy and nothing actually changed, pick a number in between you two different daily averages and call that your velocity for the load. JMO.

 

[BuzzBoss915]

Re: Chrono Question

It's all relative, people. You will not get the same readings from day to day for several reasons, one of which is ambient temperature. And, It doesn't amount to a hill of beans. Why do you need an exact number, anyway? This isn't a moonshot, is it? BB