JBM Ballistics Tables not making sense

[Knobby]

Trying to produce a chart for .308 168 Sierra Matchking.

Using BC of .462 and with G1 specified it shows I need to hold over 414 inches at 1000 yards. If I change nothing other than changing G1 to G7 it says I only need to hold over 295 inches. That's a pretty huge difference. Been trying to use thr JBM calulator because it will print a chart in MILS. ANother know any other online calulator that will produce a chart in mils?

Here's what i'm inputing:

.308
168gr
.462 bc
2600 muzzle vel.
1.5 sight height
temp 70
humd 70
alt. 500 ft
pressure 29.92

I'm a little fuzzy on the bc. Sierra gives a bunch of different values for BC. At 2600+ it's .462, .447 between 2600 and 2100 fps., etc. Which one should I be using?

 

[RobertB]

Re: JBM Ballistics Tables not making sense

Your G7 BC is only about half of what G1 is. Just use the G1 and then tweak from there. If you know what your G7 BC you can use that. I don't know what it is off the top of my head. Good luck

 

[RobertB]

Re: JBM Ballistics Tables not making sense

You use the one that your bullet speed matches. And that gets you close. If you have a range with distance, use a chart to get kindof close, then get dope at certain ranges (600 yds and 1000 yds) or what ever you can. Then juggle your MV till it gets you closer. Might have to play with B.C. a lil bit for fine tuning. Do that till you have a nice drop chart that fits your rifle,scope, loads drop. It takes a little time but will get you there soon enough.

 

[RobertB]

Re: JBM Ballistics Tables not making sense

Guess what I'm trying to say is your rifle with your load has one bullet path, That shouldn't change, so, use a chart to get you close so that you are not wasting ammo. Have a spotter out there with you so you can see your impacts. Have them stand directly behind you so they can see your trace. Then go back to the PC and play with your numbers till the ranges you have hit your target and know exactly what your real world comeup is matches what JBM gives you. With the 168 SMK if you could have 400,500,600, and 700 with a good 100 yd zero should get you a good drop chart. People here can help you farther if you have any trouble. Good luck

 

[tdow]

Re: JBM Ballistics Tables not making sense

If you want to use the G7 for the 168 SMK, enter .218. As RobertB said above, it's a totally different number, usually about half of the G1.

 

[Ring]

Re: JBM Ballistics Tables not making sense

litz G7's...
0.308 Sierra Match King 168 0.218
0.308 Sierra Match King 175 0.243
0.308 Sierra Match King 180 0.247

the data you wanted....

Range Path
(yd) (mils)
0 0
5 U8.4
10 U3.6
15 U2.2
20 U1.4
25 U0.9
30 U0.6
35 U0.4
40 U0.3
45 U0.2
50 U0.1
55 U0.1
60 0
65 0
70 0
75 0
80 0
85 0
90 0
95 0
100 0
105 0
110 0
115 0
120 U0.1
125 U0.1
130 U0.1
135 U0.1
140 U0.2
145 U0.2
150 U0.2
155 U0.3
160 U0.3
165 U0.3
170 U0.4
175 U0.4
180 U0.4
185 U0.5
190 U0.5
195 U0.5
200 U0.6
205 U0.6
210 U0.7
215 U0.7
220 U0.7
225 U0.8
230 U0.8
235 U0.9
240 U0.9
245 U0.9
250 U1.0
255 U1.0
260 U1.1
265 U1.1
270 U1.2
275 U1.2
280 U1.3
285 U1.3
290 U1.3
295 U1.4
300 U1.4
305 U1.5
310 U1.5
315 U1.6
320 U1.6
325 U1.7
330 U1.7
335 U1.8
340 U1.8
345 U1.9
350 U1.9
355 U2.0
360 U2.0
365 U2.1
370 U2.1
375 U2.2
380 U2.2
385 U2.3
390 U2.3
395 U2.4
400 U2.5
405 U2.5
410 U2.6
415 U2.6
420 U2.7
425 U2.7
430 U2.8
435 U2.8
440 U2.9
445 U3.0
450 U3.0
455 U3.1
460 U3.1
465 U3.2
470 U3.3
475 U3.3
480 U3.4
485 U3.4
490 U3.5
495 U3.6
500 U3.6
505 U3.7
510 U3.8
515 U3.8
520 U3.9
525 U3.9
530 U4.0
535 U4.1
540 U4.1
545 U4.2
550 U4.3
555 U4.3
560 U4.4
565 U4.5
570 U4.6
575 U4.6
580 U4.7
585 U4.8
590 U4.8
595 U4.9
600 U5.0
605 U5.0
610 U5.1
615 U5.2
620 U5.3
625 U5.3
630 U5.4
635 U5.5
640 U5.6
645 U5.6
650 U5.7
655 U5.8
660 U5.9
665 U6.0
670 U6.0
675 U6.1
680 U6.2
685 U6.3
690 U6.4
695 U6.4
700 U6.5
705 U6.6
710 U6.7
715 U6.8
720 U6.9
725 U7.0
730 U7.0
735 U7.1
740 U7.2
745 U7.3
750 U7.4
755 U7.5
760 U7.6
765 U7.7
770 U7.8
775 U7.8
780 U7.9
785 U8.0
790 U8.1
795 U8.2
800 U8.3
805 U8.4
810 U8.5
815 U8.6
820 U8.7
825 U8.8
830 U8.9
835 U9.0
840 U9.1
845 U9.2
850 U9.3
855 U9.4
860 U9.5
865 U9.6
870 U9.8
875 U9.9
880 U10.0
885 U10.1
890 U10.2
895 U10.3
900 U10.4
905 U10.5
910 U10.7
915 U10.8
920 U10.9
925 U11.0
930 U11.1
935 U11.2
940 U11.4
945 U11.5
950 U11.6
955 U11.7
960 U11.9
965 U12.0
970 U12.1
975 U12.2
980 U12.4
985 U12.5
990 U12.6
995 U12.8
1000 U12.9