I am trying to find out something about bullet coefficients in order to attempt to chart my bullet drop more accurately at extended range.
I am aware that a specific bullet will have a different BC which is dependent upon a number of factors such as bullet velocity, temperature, humidity, and air density (elevation).
The Sierra website shows the BC at a couple of different velocity ranges, but does not break things down any closer than that.

What I am wondering is if anybody knows or has seen any data that tells if the drop in BC is strictly linear with respect to velocity drop, or if the BC drops at a different rate than velocity.

An example of linear drop would be if the BC of a particular bullet was .5 at 3000 FPS, .49 at 2800, .48 at 2600, .47 at 2400 because the BC drops consistently with a drop in velocity.

An example of a nonlinear drop would be if the BC of a particular bullet was .5 at 3000 FPS, .49 at 2800, .47 at 2600, .44 at 2400 because the BC is dropping faster than the drop in velocity.

You can see that the second example would give considerably different bullet drop at extended range than the first example.

Hi, Bruce:
The rate of change of the ballistic coefficient is unique for each bullet. It can go up, down or sideways. Just be happy Sierra gives BCs at more than one velocity, unlike the other manufacturers. The BC of most round-noses improves quite a bit as velocity drops, while most flat-based spitizer BCs stay constant for all practical purposes. Boat-tails usually lose a bit. The Sierra manual has a long chapter on the subject.

Bye
Jack

We have done alot of testing on the 30 cal 220 and 240gr MatchKing's and the 338 300gr MatchKing.The test's they do are not Real life in the field test's.We have found the 220 has a b.c. of .655 the 240 at .711 and the 300gr at .798 to .800. across the entire velocity range.Hope I helped a little.These # were gotten by shooting at different distance's and measuring drop...

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I HUNT LONG RANGE.AND YES I USE MATCHKING'S.

Thanks, the information is interesting.
My difficulty is that I seem to be getting more bullet drop at extended range over what I am getting when calculating the BC based on muzzle velocity and bullet drop at 600 yards when sighted in for 100 yards with the 308 Win. Using the Sierra MK at 2600 fps muzzle velocity, my actual bullet drop to 600 yards is real close to what Sierra shows, but I am about 3 MOA low at 1000 yards.
Also, I am shooting 80 grain VLD Burger bullets in my 22-250 and was checking my shooting book from the last 1000 yard match and noticed that I was using a couple of MOA less adjustment than the data out to 600 yards would indicate as being correct.
At this point I am not sure if there is something inaccurate about the software or if I need to do a multi BC chart. Perhaps because of the shape the Berger seems to retain a better BC at extended range.
The printouts I am using are enough to get me on paper, but I get to shoot at extended range so seldom and change my load so often that it would be really helpful to get the two to match as closely as possible.

[This message has been edited by Bruce Gordon (edited 03-05-2002).]

Bruce...What temp you running your drop chart at?And are you shooting in the same condition's as your chart is based on?What bullet are you shooting?Give me your info.Altitude, ave Bar Pressure, ave humidity and the temp your shooting at and the bullet your shooting and I see if my data matches your's.3MOA at 1000 yard's is alot.We'll get you figured out....

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I HUNT LONG RANGE.AND YES I USE MATCHKING'S.

I will have to get the information from my shooting log this evening. I was lookin it over yesterday evening and was a bit confused.

Let me know and we'll get you fixed up....

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I HUNT LONG RANGE.AND YES I USE MATCHKING'S.

If you really want to have fun you will find out that there are different "drag curves" that can be applied. Most programs use a G1 curve but I think there are about 5 different curves that can be factored into a program. If you want you can go to "huntamerica.com" and on their web page on the left hand side is a ballistic program that you can run in basic g1 mode or you can change it to other drag curves. I think Gerard from GS customs has some of his bullets rated on the G1 Curve and some on the G5 curve and there is a pretty big difference in the flight path.
One of the writers in a recent gun magazine did some extensive testing and said that the new hornady sst and the swift scirroco seem to "outflat" their ballist coeficients pretty easily. That didn't surprise me on the scirroco but the sst doesn't strike me as being that streamlined. He said, and I believe, that it just goes to show you should do shooting out to all the ranges you might want to take game at as part of your developement.

The variability of ALL ballistic coefficients comes from the fact that they're attempts to relate the trajectory curve of, say, Bullet X, to the trajectory curve of a thoroughly researched projectile with an entirely different shape. The only projectile that flies like a G1, for example, is the G1 projectile. All others fly more or less like a G1 at certain velocities. None is linear, if I understand "linear" correctly.

Shoot some tests, do some figgerin', and take an average. No figure in a book or a software program is going to give you a precise prediction.

Also remember that "trajectory" is very general, just as ballistic coefficient is. Ten bullets from the same load and barrel won't fly exactly the same -- even if you could eliminate aiming error and were shooting a perfect rifle, you wouldn't get a one-hole group at 1,000 yards.

Bruce,
At a thousand yards (even at 600 yards) a small updraft (heat rising from the ground) can affect the flight of a bullet more than an error in BC of .05 and the same goes for cool air 'falling' on the range or head or tail winds. Three inches at 1000 yards is only .3 MOA and there are a lot of variables that can add and subtract from trajectory. That's why it's always prudent to sight in for the range you will be shooting and under the conditions you will be shooting at.

PaulS

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stay safe and live long!

Bruce

Take a look at April 2002 Shooting Times. Rick Jamison wrote a very informative article on just what you are looking for.

I have several Powley basic programs and a few other computer programs I've used over the years to calculate BC and drop. They all seem to use the G1 or G7 tables and an estimate of point shape. They all fall short. Actual target data is the most accurate, but only for that bullet and those conditions it was shot in. You can extrapolate and interpolate and nit pick but once the shot is off you can't repeat, exactly, the same conditions, exactly. The best you can expect is to get good empirical data you can trust and trust it to repeat. It is amazing what the new crop of components can do and the actual BC's calculated from the measured drop and chrono'd velocity are better than the published data.

Thanks for some more interesting information.
BTW, the difference in point of impact was 3 MOA, not 3", which is 30" at 1000 yards and happened in the following manner.
I shoot a couple of times a week at 600 yards, weather permitting, so have extremely good bullet drop dope on that yardage. Knowing the chronograph readings for my selected load in my two target rifles, I plugged all the stuff in to the software program (Load on a Disk, I think)to get a projected scope setting for the 1000 yard match. Did this with both target rifles and felt pretty good about the result.
When I shot in the match and used the initial calculated scope settings, one rifle was off by 3 MOA (30")and the other was off by 1 3/4 MOA (18")vertically. We won't get into the 15-25 mph gusting wind switching from 5 oclock to 9 oclock cause that was a whole different problem.
Both settings were on paper, so it wasn't any big deal, but I got curious as to the reason. It sounds like the reason is probably a combination of my approx. data input and inaccuracies in the software calculations and varying bullet BC with range and velocity.

Sorry, but I was not able to take the time to look over my shooting log book yesterday evening cause things were a bit hectic at home. My uncle passed away yesterday, so I was on the phone most of the evening talking to numerous relatives wishing to be the first to pass on the news and make sure that I am willing to be a pall bearer.