I have read that when the chrono is placed 15 feet from the muzzle, add 20-25' for muzzle velocity. Is this correct? If so, how much should be added for 15 metres?

Kevin

I think your 20-25 is a touch high.

Here is a 165gr nosler starting at 3000fps as it is adjusted back from a normal reading distance. If you notice the program used 10' as the Chrony distance and adjusted the velocity up 7fps as a muzzle velocity.

 Trajectory 
Input Data 
Manufacturer:  Nosler  Description:  Accubond™ Spitzer  
Caliber:  0.308 in  Weight:  165.0 gr  
Ballistic Coefficient:  0.475 G1 (ASM)    
 
Muzzle Velocity: 3000.0 ft/s Distance to Chronograph: 10.0 ft 
 
Sight Height: 1.50 in Sight Offset: 0.00 in 
Zero Height: 0.00 in Zero Offset: 0.00 in 
Windage: 0.000 MOA Elevation: 0.000 MOA 
Line Of Sight Angle: 0.0 deg Cant Angle: 0.0 deg 
 
Wind Speed: 10.0 mph Wind Angle: 90.0 deg 
Target Speed: 10.0 mph Target Angle: 90.0 deg 
Target Height: 12.0 in   
 
Temperature: 59.0 °F Pressure: 29.92 in Hg 
Humidity: 0 % Altitude: 0.0 ft 
 
Vital Zone Radius: 5.0 in   
 
Std. Atmosphere at Altitude: No Pressure is Corrected: Yes 
Zero at Max. Point Blank Range: No Target Relative Drops: Yes 
Mark Sound Barrier Crossing: No Include Extra Rows: No 
Column 1 Units: 1.00 in Column 2 Units: 1.00 MOA 
Round Output to Whole Numbers: No   
Output Data 
Elevation: 3.357 MOA Windage: 0.000 MOA 
 
Atmospheric Density: 0.07647 lb/ft³ Speed of Sound: 1116.4 ft/s 
 
Maximum PBR: 361 yd Maximum PBR Zero: 307 yd 
Range of Maximum Height: 169 yd Energy at Maximum PBR: 1939.4 ft•lbs 
 
Sectional Density: 0.248 lb/in²   

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 -1.5 *** 0.0 *** 3007.1 2.693 3312.4 0.000 0.0 *** 
1 -1.5 -139.9 0.0 0.0 3005.0 2.692 3307.7 0.001 0.2 16.8 
2 -1.4 -68.3 0.0 0.0 3002.8 2.690 3303.0 0.002 0.4 16.8 
3 -1.4 -44.4 0.0 0.0 3000.7 2.688 3298.4 0.003 0.5 16.8 
4 -1.4 -32.5 0.0 0.0 2998.6 2.686 3293.7 0.004 0.7 16.8 
5 -1.3 -25.4 0.0 0.0 2996.5 2.684 3289.0 0.005 0.9 16.8 
6 -1.3 -20.6 0.0 0.0 2994.3 2.682 3284.4 0.006 1.1 16.8 
7 -1.3 -17.2 0.0 0.0 2992.2 2.680 3279.8 0.007 1.2 16.8 
8 -1.2 -14.7 0.0 0.0 2990.1 2.678 3275.1 0.008 1.4 16.8 
9 -1.2 -12.7 0.0 0.1 2988.0 2.676 3270.5 0.009 1.6 16.8 
10 -1.2 -11.2 0.0 0.1 2985.9 2.674 3265.9 0.010 1.8 16.8 
11 -1.1 -9.9 0.0 0.1 2983.8 2.673 3261.2 0.011 1.9 16.8 
12 -1.1 -8.8 0.0 0.1 2981.7 2.671 3256.6 0.012 2.1 16.8 
13 -1.1 -7.9 0.0 0.1 2979.5 2.669 3252.0 0.013 2.3 16.8 
14 -1.0 -7.1 0.0 0.1 2977.4 2.667 3247.4 0.014 2.5 16.9 
15 -1.0 -6.5 0.0 0.1 2975.3 2.665 3242.8 0.015 2.6 16.9 
16 -1.0 -5.9 0.0 0.1 2973.2 2.663 3238.2 0.016 2.8 16.9 
17 -1.0 -5.4 0.0 0.1 2971.1 2.661 3233.6 0.017 3.0 16.9 
18 -0.9 -4.9 0.0 0.1 2969.0 2.659 3229.0 0.018 3.2 16.9 
19 -0.9 -4.5 0.0 0.1 2966.9 2.657 3224.5 0.019 3.4 16.9 
20 -0.9 -4.2 0.0 0.1 2964.8 2.656 3219.9 0.020 3.5 16.9 
21 -0.8 -3.9 0.0 0.1 2962.7 2.654 3215.3 0.021 3.7 16.9 
22 -0.8 -3.6 0.0 0.1 2960.6 2.652 3210.8 0.022 3.9 16.9 
23 -0.8 -3.3 0.0 0.1 2958.5 2.650 3206.2 0.023 4.1 16.9 
24 -0.8 -3.1 0.0 0.1 2956.4 2.648 3201.7 0.024 4.3 16.9 
25 -0.7 -2.8 0.0 0.1 2954.3 2.646 3197.1 0.025 4.4 16.9 
26 -0.7 -2.6 0.0 0.2 2952.2 2.644 3192.6 0.026 4.6 16.9 
27 -0.7 -2.4 0.0 0.2 2950.1 2.642 3188.1 0.027 4.8 16.9 
28 -0.7 -2.3 0.0 0.2 2948.0 2.641 3183.5 0.028 5.0 16.9 
29 -0.6 -2.1 0.1 0.2 2945.9 2.639 3179.0 0.029 5.1 16.9 
30 -0.6 -2.0 0.1 0.2 2943.8 2.637 3174.5 0.030 5.3 16.9 
31 -0.6 -1.8 0.1 0.2 2941.7 2.635 3170.0 0.031 5.5 17.0 
32 -0.6 -1.7 0.1 0.2 2939.6 2.633 3165.5 0.032 5.7 17.0 
33 -0.6 -1.6 0.1 0.2 2937.5 2.631 3161.0 0.033 5.9 17.0 
34 -0.5 -1.5 0.1 0.2 2935.5 2.629 3156.5 0.034 6.0 17.0 
35 -0.5 -1.4 0.1 0.2 2933.4 2.627 3152.0 0.035 6.2 17.0 
36 -0.5 -1.3 0.1 0.2 2931.3 2.626 3147.5 0.036 6.4 17.0 
37 -0.5 -1.2 0.1 0.2 2929.2 2.624 3143.0 0.037 6.6 17.0 
38 -0.4 -1.1 0.1 0.2 2927.1 2.622 3138.6 0.038 6.8 17.0 
39 -0.4 -1.0 0.1 0.2 2925.0 2.620 3134.1 0.039 6.9 17.0 
40 -0.4 -1.0 0.1 0.2 2922.9 2.618 3129.6 0.040 7.1 17.0 
41 -0.4 -0.9 0.1 0.2 2920.9 2.616 3125.2 0.042 7.3 17.0 
42 -0.4 -0.8 0.1 0.3 2918.8 2.614 3120.7 0.043 7.5 17.0 
43 -0.4 -0.8 0.1 0.3 2916.7 2.612 3116.3 0.044 7.7 17.0 
44 -0.3 -0.7 0.1 0.3 2914.6 2.611 3111.8 0.045 7.8 17.0 
45 -0.3 -0.7 0.1 0.3 2912.5 2.609 3107.4 0.046 8.0 17.0 
46 -0.3 -0.6 0.1 0.3 2910.5 2.607 3103.0 0.047 8.2 17.0 
47 -0.3 -0.6 0.1 0.3 2908.4 2.605 3098.5 0.048 8.4 17.0 
48 -0.3 -0.5 0.1 0.3 2906.3 2.603 3094.1 0.049 8.6 17.1 
49 -0.2 -0.5 0.2 0.3 2904.2 2.601 3089.7 0.050 8.8 17.1 
50 -0.2 -0.4 0.2 0.3 2902.2 2.599 3085.3 0.051 8.9 17.1 
51 -0.2 -0.4 0.2 0.3 2900.1 2.598 3080.9 0.052 9.1 17.1 
52 -0.2 -0.4 0.2 0.3 2898.0 2.596 3076.5 0.053 9.3 17.1 
53 -0.2 -0.3 0.2 0.3 2896.0 2.594 3072.1 0.054 9.5 17.1 
54 -0.2 -0.3 0.2 0.3 2893.9 2.592 3067.7 0.055 9.7 17.1 
55 -0.2 -0.3 0.2 0.3 2891.8 2.590 3063.3 0.056 9.8 17.1 
56 -0.2 -0.3 0.2 0.3 2889.8 2.588 3059.0 0.057 10.0 17.1 
57 -0.1 -0.2 0.2 0.3 2887.7 2.586 3054.6 0.058 10.2 17.1 
58 -0.1 -0.2 0.2 0.3 2885.6 2.585 3050.2 0.059 10.4 17.1 
59 -0.1 -0.2 0.2 0.4 2883.6 2.583 3045.9 0.060 10.6 17.1 
60 -0.1 -0.2 0.2 0.4 2881.5 2.581 3041.5 0.061 10.8 17.1 
61 -0.1 -0.1 0.2 0.4 2879.4 2.579 3037.1 0.062 10.9 17.1 
62 -0.1 -0.1 0.2 0.4 2877.4 2.577 3032.8 0.063 11.1 17.1 
63 -0.1 -0.1 0.3 0.4 2875.3 2.575 3028.5 0.064 11.3 17.1 
64 -0.1 -0.1 0.3 0.4 2873.3 2.574 3024.1 0.065 11.5 17.2 
65 -0.1 -0.1 0.3 0.4 2871.2 2.572 3019.8 0.066 11.7 17.2 
66 -0.0 -0.1 0.3 0.4 2869.1 2.570 3015.5 0.067 11.9 17.2 
67 -0.0 -0.0 0.3 0.4 2867.1 2.568 3011.1 0.068 12.0 17.2 
68 -0.0 -0.0 0.3 0.4 2865.0 2.566 3006.8 0.070 12.2 17.2 
69 -0.0 -0.0 0.3 0.4 2863.0 2.564 3002.5 0.071 12.4 17.2 
70 -0.0 -0.0 0.3 0.4 2860.9 2.563 2998.2 0.072 12.6 17.2 
71 -0.0 -0.0 0.3 0.4 2858.9 2.561 2993.9 0.073 12.8 17.2 
72 0.0 0.0 0.3 0.4 2856.8 2.559 2989.6 0.074 13.0 17.2 
73 0.0 0.0 0.3 0.4 2854.8 2.557 2985.3 0.075 13.2 17.2 
74 0.0 0.0 0.3 0.4 2852.7 2.555 2981.0 0.076 13.3 17.2 
75 0.0 0.0 0.4 0.5 2850.7 2.553 2976.8 0.077 13.5 17.2 
76 0.0 0.0 0.4 0.5 2848.6 2.551 2972.5 0.078 13.7 17.2 
77 0.0 0.0 0.4 0.5 2846.6 2.550 2968.2 0.079 13.9 17.2 
78 0.0 0.0 0.4 0.5 2844.5 2.548 2964.0 0.080 14.1 17.2 
79 0.0 0.0 0.4 0.5 2842.5 2.546 2959.7 0.081 14.3 17.2 
80 0.0 0.0 0.4 0.5 2840.4 2.544 2955.4 0.082 14.5 17.3 
81 0.0 0.0 0.4 0.5 2838.4 2.542 2951.2 0.083 14.6 17.3 
82 0.0 0.0 0.4 0.5 2836.4 2.541 2946.9 0.084 14.8 17.3 
83 0.0 0.0 0.4 0.5 2834.3 2.539 2942.7 0.085 15.0 17.3 
84 0.0 0.0 0.4 0.5 2832.3 2.537 2938.5 0.086 15.2 17.3 
85 0.0 0.0 0.5 0.5 2830.2 2.535 2934.2 0.087 15.4 17.3 
86 0.0 0.0 0.5 0.5 2828.2 2.533 2930.0 0.088 15.6 17.3 
87 0.0 0.0 0.5 0.5 2826.2 2.531 2925.8 0.090 15.8 17.3 
88 0.0 0.0 0.5 0.5 2824.1 2.530 2921.6 0.091 15.9 17.3 
89 0.0 0.0 0.5 0.5 2822.1 2.528 2917.4 0.092 16.1 17.3 
90 0.0 0.0 0.5 0.5 2820.1 2.526 2913.2 0.093 16.3 17.3 
91 0.0 0.0 0.5 0.6 2818.0 2.524 2909.0 0.094 16.5 17.3 
92 0.0 0.0 0.5 0.6 2816.0 2.522 2904.8 0.095 16.7 17.3 
93 0.0 0.0 0.6 0.6 2814.0 2.520 2900.6 0.096 16.9 17.3 
94 0.0 0.0 0.6 0.6 2811.9 2.519 2896.4 0.097 17.1 17.3 
95 0.0 0.0 0.6 0.6 2809.9 2.517 2892.2 0.098 17.3 17.3 
96 0.0 0.0 0.6 0.6 2807.9 2.515 2888.1 0.099 17.4 17.4 
97 0.0 0.0 0.6 0.6 2805.8 2.513 2883.9 0.100 17.6 17.4 
98 0.0 0.0 0.6 0.6 2803.8 2.511 2879.7 0.101 17.8 17.4 
99 0.0 0.0 0.6 0.6 2801.8 2.510 2875.6 0.102 18.0 17.4 
100 -0.0 -0.0 0.6 0.6 2799.8 2.508 2871.4 0.103 18.2 17.4 
          

Well crap for some reason "code" in brackets didn't hold the table. Anyway first number is yds from muzzle and 6th is the velocity.

I used http://www.jbmballistics.com/b...rs/calculators.shtml

To quote the Queen, "What difference does it make?"

Exactly; I never measure the distance; I set my Chrony out in front of the muzzle; How far? who cares? The only thing is that you want it far enough away so you don't measure muzzle blast/gas, instead of bullets, I think I read somewhere.
Point is, it don't matter to me.

Bar bets!

Thank you everyone. I set the distance to stop muzzle blast being a problem and measuring the same everytime for consistency.
I read it in a few different books over time and really wondered ,how that figure is arrived at.

Set mine with the first screen 10ft from the muzzle and the second at 20ft so it's measured at 15ft form the muzzle. 10 to 20fps or whatever isn't going to make any difference in calculating ballistics or pressure.

Thank you Blacktailer. Nice setup! I like how you have the timber across the front of screens, I thought of doing something similar in the hope of stopping the blast from interfering with the readings and moving the chrono closer to the muzzle.
I suppose I didn't word my question right as what I was trying to ask, was, how do they work out what they call actual muzzle vel. from the chrono reading in regard to how far from the muzzle?

Kevin

I just set up my Chrony to be out from the roof of the shooting range tables and close enough so I can read the screen.

Used to set it up much closer for pistol shooting and also for shotgun so the wad was still intact as it passed through the guides. Always got consistent results even with the shotgun blast.

A few feet/sec MV here or there makes no difference, it is consistent results that matter.

quote:

A few feet/sec MV here or there makes no difference, it is consistent results that matter.

All well and good. So, no one can answer my question?

quote:

All well and good. So, no one can answer my question

Do I know for sure NO! But, I'd bet you a that it is simply calculated.

As in my example I posted. The ballistic software assumed when I said 3000fps that was at 10' normal distance to a chrono. The calculated back. Just like downstream velocity is calculated based on the BC of the bullet.

If you really want to be accurate measure velocity at 10' move the screens down to 10-20yds or even better 50-100yds. Re-measure the velocity then ratio the reduction to adjust back to muzzle.

Hard to imagine a screen type that could handle the muzzle blast at ground zero.

Hey maybe the new radar type will.

Thank you Paul. I like that idea.
Long way to come for a cup of coffee.
I expessed an interest in the Labradar last year,so we'll see when they are sorted.

Kevin

quote:

Originally posted by cha:
Thank you everyone. I set the distance to stop muzzle blast being a problem and measuring the same everytime for consistency.
I read it in a few different books over time and really wondered ,how that figure is arrived at.

If you are OCD (like me) then you just like to know stuff like that. I use JBM Ballistics and enter the distance to the skyscreens. JBM then calculates the actual muzzle velocity. Usually it's only 6 - 10 fps.

http://www.jbmballistics.com/c...bmtraj_drift-5.1.cgi

Thank you Fjold. That is the same one Ramrod put up for me, you're right, I just want to know.
Thanks everyone for your responses, appreciated.

All the best

Kevin

Hey, CHA... Wouldn't the Magnetospeed satisfy your quest...