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Help calculating a BC from two speeds

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Unread 10-03-2007, 09:44 PM
Silver Member
Join Date: Oct 2007
Posts: 165
Tom, I think most of us have been talking about G1 models. Using the JBM conversion your estimate of .340 in a G5 is about a .559 as a G1. So I guess you are fairly close with the estimated coeffecient. I guess it just matters wat drag model to apply. I like the idea of using the G5 or maybee the G7 when developing drop data. I have heard from several people that it really doesn't make much difference until you get to transonic speeds. I will run numbers on both darg models and check to see the difference at 1000 yards. It has been too windy the past couple days to shoot at 800 yards to verify the drop data, but will post an update as soon as I do.

It sure is nice to be a part of a forum that has educated/experienced people that can help when I have questions like this. Thanks to all who participate!

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Unread 10-03-2007, 11:57 PM
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Join Date: Jan 2002
Location: Wenatchee, WA
Posts: 806

As I said before, I haven't ever quite got around to calculating BCs from speed or drops; normally I've been pretty satisfied w/ the results from published G1 BCs + Exbal + Kestrel...

That said, I've always kind of wanted to, just to see what kind of numbers I do get. Probably one of those 'curiosity killed the cat' things, but I'm a sucker for those

If you don't mind me asking... did you use two chronographs or just one?

If you used two chronographs, how accurately did you measure your distances from the muzzle?

Did you shoot thru both units back to back first to see how much they differed?

The reason I ask is I have a CED Millenium (nice unit) and one of my friends has a similar unit. Note I didn't say 'identical'... I've borrowed it a few times to check mine, and one of them (I think maybe mine?) reads about 25-30fps lower than the other (depends which day, load, phase of the moon as to exactly 'how much lower').

If we did your test using these two chronographs... and the far unit was the one that reads say, 25 fast (i.e. down range velocity is really 2750fps), your computed G1 BC is something like 0.480.

If instead the *near* chrono was the one that reads 25fps fast (i.e. actual velocity was 3070fps), your computed G1 BC works out to 0.561.

I have no idea as to how to determine which chrono is the 'right' one. Some have suggested shooting match grade .22LR ammo over it which should chrono at a very consistent speed of something like 1030fps or somewhere in there. The question is, if I'm 'calibrating' my chrono using ammo that is going roughly 1/3 as fast as what I actually want to test... is my expected error 1/3 the size (~8fps) as well? Saying that chrono A reads 8fps faster @ ~1000fps than chrono B is going to be very interesting to see if you can even see that consistently.

I've probably opened a bigger can of worms than I should have here, and I'm not attempting to start a pissin' contest w/ anyone... more just kind of thinking 'out loud' about what all to consider when judging the validity of some of our 'tests'. For that matter, it's getting late, and I could be completely off-base here and just need 'calibrated' with a clue-by-four... wouldn't be the first and most certainly not the last time for that! ;)

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Unread 10-04-2007, 07:25 AM
Silver Member
Join Date: Oct 2007
Posts: 165
I actually used one chronograph. I have a CED M2. What I did was shoot a five shot group with the chrono at "about" 10 feet, and then moved the chrono to the target at 200 yards. I then shot another 5 shot group. Then shot one more 5 shot group with the chono left at 200 yards. I only got 7 readings with the chrono at 200 yards, 3 the frist time and 4 the second after a sight adjustment. The wind was blowing at me about 10-15 MPH from about 75deg out of the left. I had a couple shots that must have missed the "beam" of the chrono, but all still went through the screens. I then brought the chrono bach to 10 feet and shot another 5 shot group. Then took the averages of the 10 up close and the 7 far away. My extreme spread was 42fps up close, and 46fps at the longer range.

Obviosly there is some room for error with the variations fps of the load and distance between the chronos. However the gun is only shooting about 1 moa groups , and at 800 yards there is also error from bullet impact within the groupiong. Not to include human error (which there is a lot of with me ;))

So for now, I am looking a speed derived BC of about .530. It looks like the weather will cooperate on Friday, and I will see what the shot BC mathes these findings.
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Unread 10-04-2007, 08:39 AM
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Join Date: Jan 2004
Location: Blackfoot, Idaho
Posts: 8,826

I too get hooked on this stuff. I also used G5 so I don't feel so bad about my numbers.

A hint, Use the borrowed chrono @ distance;) I was a bit conservative on my first shot (limited confidence) and whacked the front shade on the first shot.:o

To check the two chonos I but one against the other @ 10' and shoot a few through them.

It's kind of interesting but I'm finding that "other" ways of determining actual bc are more rewarding.
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!
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Unread 10-05-2007, 08:11 AM
Silver Member
Join Date: Oct 2007
Posts: 165
I got a chance to shoot last night at 800 yards. It appears that I was hitting about 4" low with the group average. It was about 20 degrees cooler than when I sighted in and the chart I had was for 60 degrees, (it was about 40 degrees last night). The BC I used for this table was .530, and with the cooler weather the 4" low should be about right. My guess is I might give .525 a try.

I guess it is all for not though as it is not grouping very well at all. I had a friend tell me that these very low drag bullets sometimes don't group well at closer ranges, but stabalize and group well at longer ranges. In this case it is not true. It was grouping about 1.75-2.00" at 200 yards, and last night my 7 shot group at 800 yards was about 16-17". I would estimate my actual hold at that distance to be about 4", so you could subtract that from the group size, but it is still way over 1 moa. I am sure I could do a little more with my load work-up (seating depth, powder charge, etc.) but it seems that the bullets are getting worse at longer ranges.

My Nolser 140 accubond load I have been using averages about .5to .6 moa up close (100-200), and 1 moa at the longer distances (600-800). The BC I use with this bullet is about .490. I was really hoping that the wlidcat bullet would group so I could have a heavier better BC bullet, but I am not going to give up much accuracy to get it. The other reason I wanted to switch bullets is that the accubond is so hard and well built that I am blowing through my deer at 600 yards with small exit holes. On a chest hit the damage is not as much as I want.

I have been waiting for Berger to put out a 270 bullet for over 6 months, and they keep pushing back the release date. I might give it a try when the do finish them I guess.

If anyone wants to try the .277 ULD rebated Boattail bullet I have an extra box of 100 that might be for sale... LOL

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