I am using Sierra's Infinity software to figure drop and drift, and I'm having trouble getting realistic results because of the different manufacturers bc's. Is there a chart somewhere in cyber-land that gives calculated bc's similar to Sierra's for the major bullet makers?
An example of what I am having trouble with is Speer's 6mm 80sp with a bc of .365 across the board, while Sierra's 85 hpbt with figures of .282-.311. If calculated at the same speed, it gives the Speer a lot less drift than the Sierra. Obviously, this should not be the case. If I had the calculated bc's for the Speer, I could get realistic numbers.
I'm hoping that there is something out there that will give me some good bc's to plug in to my software. Thanks, Jeff.

I do not know where the data is available in cyberland. In the Speer #13 Reloading Manual, on page 689 in the Long Range Tables, is the exterior ballistics table for BC of 0.36. Find the velocity of your load in the tables and the ballistic data is there for ranges out to 500 yards.

I think your calculated result is as it should be.

Wind drift is NOT mostly caused by the wind blowing against the side of the bullet.

It is mostly caused because the bullet will slightly "nose into" the wind, so that net airflow is directly from nose to tail of the bullet. The drag vector points straight out the tail of the bullet, so that vector can be resolved into two vectors, one pointing parallel to the flight path, and one in the direction of the crosswind. It is that crosswind vector component that mainly causes deflection.

The higher the BC, the less the deflection.

Lee; Those are the figures that the software is using, .365 to be exact.

denton, thanks for the explaination, it sounds very reasonable. As you stated, as the bc gets "higher", there will be less deflection. That is where I believe the error is coming from. Just from a logical standpoint, there is no way that the 80 sp is giong to drift less than the 85 hpbt, and the reason the software says it will is because of the difference in the bc values. For a 90* 25 mph crosswind, it gives the Speer a drift at 400 of 28.45 in. and the Sierra 41.18! Even the drop is almost 8 1/2 in. less. There is no way those are believable figures. That hpbt is going to outperform the sp at that distance. When I change the bc of the Speer to .280 to match the low number of the Sierra, it changes the drift to 38.91, which is closer to what I would expect. This is why I would like to get actual calculated numbers to match Sierra's instead of what I assume is an averaged "generic" figure. Is my logic acceptable, or am I missing something?

Having not read your post as carefully as I should have, I ran your bullets through the RSI Shooting Lab, but used a 10 mph crosswind, instead of your 25 mph. I assumed 3100 fps.

The Speer 80 SP, with a BC of .365, has a deflection at 500 yards of 23.35". The Sierra 85 HPBT, with a BC of .282, shows a deflection of 32.66".

Higher BC, less deflection.

I have sometimes wondered if the Speer BC numbers are a little inflated. Their bullets seem to have the same shape as Sierra, but their BC's run higher. Hornady bullets have a little different shape, and generally lower BC's. Maybe there is some subtle difference I am missing...

The BC of a bullet can vary a little depending on which form factor is used in the calculation as a "standard projectile". But as far as I know the common hunting bullets are using similar calculations to determine the BC.

The common balistics software uses BC's as supplied by the different manufacturers. I have Lee Shooter, RCBS load 2.88, and the RSI shooting lab demo on my computer. But I often go back to the tables in the reloading manuals for comparison purposes.

If you look at the Sieera manual, or at your Infinity software you will see that the Sierra 85 gr SPT has a BC of .329 compared to .279 for the Sierra 85 gr HPBT. The former bullet here is not even a boat tail. Compare this to a BC of .445 for the 100 gr Sierra 243 SPBT.

The 85 gr Sierra HPBT is simply a very high drag bullet compared to other bullets in its class.

I will look at both of the bullets you mentioned in their respective reloading manual's balistic charts using 3200 fps as a practical MV from a 243 Win.
If we sight both bullets to zero at 300 yds the chart says the 85 gr Sierra HPBT drops 31.8 inches at 500 yd with 60.5 inches of deflection from a twenty mph sidewind.

The Speer # ten manual say that their bullet with a BC of 360 will drop 27.6 inches at 500 yds from a 300 yd zero, but does not list wind deflection.

These numbers should not be much different than what you would obtain from the Infinity program.

I sure hope this helps.

Thanks for your input guys. What I am seeing here is when I look at the physical bullets themselves, not a statistic, I can see that there is no way that that the flat-based Speer has a higher coefficient of form. Maybe I should have compared apples to apples, so to speak, and used Sierra's 80sp to begin with. When I plug it in, it has a bc of only .288 compared to the Speer of .365. I guess what I am saying is that I think Speer's numbers are full of crap! When you look at both bullets, there is not that big a difference in their shape. If I plug in a Horn. 87, and a Nosler 80, I get similar results, although with their higher published bc's they graph closer than the Sierra. I stil don't think there is enough difference in their shapes to warrant a large spread in the bc. I would think that they all would fly pretty much the same, but the chart shows that large diff. just because of the numbers. If I were actually shooting these bullets, I would be very surprised to find that the Speer drifts 8 1/2 in. less than the Sierra. That's why I would like to find some numbers to plug into these other bullets to give a more realistic comparison. I think that Sierra actually handicaps itself by giving better (more accurate?) numbers for their bullets than the other makers. Not to say anyone is wrong, just that Sierra has a little different priority than the others, in that they put more effort in ballistics, whereas the others put more effort in terminal performance on game. I wish I had the time and money to actually do all of the shooting required to find the actual results, but I will have to settle for the statistical kind for now. Jeff

If you look at the components of the Ingalls calculations you find that the meplat is most important and that accounts for the facts in the Sierra data.

As to BC data in general it's quite discouraging as the various manufacturers do not have believable numbers. As noted before the Speer data does seem high.

I believe it was POP who provided data from a magazine on .30 caliber BC's and most all of them were high as compared to the actual velocities at the muzzle and downrange from chronographed data.

I use an Oehler model 43PBL with acoustic target. It calculates BC for every shot. And the BC changes with speed. Use a Barnes X 85 grain .257" as example.

3311fps = 0.370BC

3267fps = 0.349BC

Lots of input to calculate BC, such as humidity, baro pressure, altitude, temp, etc. BC moves around due to changing conditions. Listed numbers, where ever they are found, by nature are a best guess. Buy a 43PBL and measure your own. Then you'll know exactly. No guesswork.

quote:

Originally posted by onefunzr2:
I use an Oehler model 43PBL with acoustic target. It calculates BC for every shot. And the BC changes with speed. Use a Barnes X 85 grain .257" as example.

3264fps = 0.147BC

3253fps = 0.142BC

Lots of input to calculate BC, such as humidity, baro pressure, altitude, temp, etc. BC moves around due to changing conditions. Listed numbers, where ever they are found, by nature are a best guess. Buy a 43PBL and measure your own. Then you'll know exactly. No guesswork.

Dave,

I don't find a 85 gr Barnes bullet on their site but could a 85 gr .257 bullet have that low a BC as .14 ?

Thanks for the replies. It didn't help a helluva lot, but I find it a very interesting topic I had kind of surmised that the bc was kind of a ball-park figure especially since it changes with velocity. I just didn't realize it would make that much different in the stats. I don't shoot any Sierra's in my rifles at the moment, but my cousin gave me this program and I started plugging in numbers the other day and they weren't making much sense. It will give me a general idea of what to expect though. Thanks, Jeff.

BC estimator

JeffP40,

Lets look at this question from another perspective.

Sierra lists their 243 cal 85 gr spitzer flat base bullet with a BC of .329 and their 100 gr SPBT at .445.

Speer lists their 243 cal 80 gr spitzer flat base with a BC of .365 and their 85 gr spitzer boat tail at .404.

When comparing these four bullets, the bc's seem to me to be perfectly reasonable. If the Speer bullets have a bc of believable value when compared to the Sierra spitzer boat tail, it is reasonable to assume that the value is also believable when compared to the Sierra HPBT. And if Sierra says their HPBT bullet has a lower BC than the equivalent Sierra flat base, why would it not also be lower than the Speer bullet.

Now if we return the discussion to your bullet of interest, the Sierra 85 gr HPBT, Sierra says it has a BC of lower value than their 85 gr flat base. I have Sierra manuals dating back over 20 years, the new and old manuals agree on this matter.

Apparently the Sierra 85 gr HPBT was built for reliable expansion rather than long range balistic performance. If you look through the Sierra manual you will see this trend continues with many of their HPBT bullets designed for hunting.

The .224 55 gr HPBT is listed @ .195 vs .299 for their 55 gr SBT.

The .257 90 gr HPBT is listed @ .276 vs .303 for the 87 gr SPT (flat base).

The .277 140 gr HPBT is listed at .354 vs .479 for the 140 gr SBT.

A large hollow point cavity seems to have a negative effect on balistic coefficient.

This trend does not hold true with Sierra's HPBT Matchking bullets. They are designed with very high BC's.

It is my opinion that any discrepancies between the BC's listed from major bullet manufacturers are inconsequential.

If you still do not trust the data provided to you from Infinity, test it. Forget wind drift for testing of BC. Wind is too unpredictable to get consistant shot to shot values. You would need to shoot hundreds? of rounds to average out shot to shot variations in wind velocity. (Denton, if he is still watching this thread, could tell us exactly how many rounds of each bullet would need to be fired to give us data of statistical significance.)

Instead look only at balistic arc. You only need an accurate rifle and a cheap Chrony to make a load with each bullet to the same muzzle velocity. On a calm day, zero each bullet at 100 yds and measure drop at 400 yd. Compare your observed results to the charts prepared with Infinity or those at the back of the manuals.

Unless you are using competitive bench rest equipment, I would not go beyond 400 yds as group dispersion will begin to make drop hard to read. But if you can hold ten shots from each rifle within a three or four inch group at four hundred yards, you should be able to closely compare balistic coefficient.

Under these conditions, according to their respective balistic charts, the 243 cal Speer 80 gr spitzer bullet will drop 23.6 in. at 400 yds and the Sierra 85 gr HPBT will drop 26.1 in. at 400 yds.

You'll find a list of BCs @ Steve's page and at
http://www.nfa.ca/nfafiles/cfjarchive/ballistics/C243 .
But this will not be of much use to you as these lists contain the data received from the manufacturers. (If you feel Speer's BC of .365 is a bit high, have a look at Federal's .243 80gn BTHP with a BC of .405 !)

A while ago I made a comparison of several ballistics calculators and found their results to be mostly the same -except Sierra.


This is, imho, no reason to stay away from Sierra as they seem to spend a huge amount of work in their ballistics research.
As they have such a wide range of well documented bullets, it's adviseable to make them your bullets reference.

To do this, you need to shoot a set (of 5) of Sierra bullets at the distance(s) of interest and note group center and average Vo.
Now load 5 rounds with your bullet X and do the same. The new group usually prints higher or lower. Measure the distance of the two groups.
If you shoot both bullets with the same Vo, it's only a little BC-fiddling with your software to find "your BC" for bullet X.
Make sure you have the same range conditions and be prepared to find different BCs for different shooting distances.

Well spotted, Savage 99. You are of course correct, a BC of .14 is too low for that bullet. My mistake. The figures have been corrected but my point remains: BC will vary with bullet speed. It is not carved in stone.

Barnes discontinued the 85 some years ago in favor of the 90 grain X bullet.

quote:

Originally posted by JeffP40:
Thanks for your input guys. What I am seeing here is when I look at the physical bullets themselves, not a statistic, I can see that there is no way that that the flat-based Speer has a higher coefficient of form. Maybe I should have compared apples to apples, so to speak, and used Sierra's 80sp to begin with. When I plug it in, it has a bc of only .288 compared to the Speer of .365. I guess what I am saying is that I think Speer's numbers are full of crap! When you look at both bullets, there is not that big a difference in their shape. If I plug in a Horn. 87, and a Nosler 80, I get similar results, although with their higher published bc's they graph closer than the Sierra. I stil don't think there is enough difference in their shapes to warrant a large spread in the bc. I would think that they all would fly pretty much the same, but the chart shows that large diff. just because of the numbers. If I were actually shooting these bullets, I would be very surprised to find that the Speer drifts 8 1/2 in. less than the Sierra. That's why I would like to find some numbers to plug into these other bullets to give a more realistic comparison. I think that Sierra actually handicaps itself by giving better (more accurate?) numbers for their bullets than the other makers. Not to say anyone is wrong, just that Sierra has a little different priority than the others, in that they put more effort in ballistics, whereas the others put more effort in terminal performance on game. I wish I had the time and money to actually do all of the shooting required to find the actual results, but I will have to settle for the statistical kind for now. Jeff

It seems to me that you are looking at both bullets, and concluding, probably wrongly, that the Speer's ballistic coefficient cannot be higher than the Sierra's based on their physical appearance!! However, the only way to test this for certain is to shoot both bullets at as close to the same MV as possible, measure that velocity at, say, 10 yards, then measure their velocities again down range, at perhaps 110 yards, and see how much initial measured velocity each bullet lost travelling 100 yards. Then you match these loss figures to the known B.C. of a bullet that loses the same velocity travelling the same distance.

Estimating the B.C. of a bullet by calculations may come close, but the only way of knowing the ballistic coefficient of a given bullet for certain is by conducting firing tests of that bullet. This requires the capability to measure retained velocity at various ranges, or a doppler-effect type radar chronograph that measures the velocity of the projectile throughout its travel, such as the Army uses for testing artillery projectiles.

Idaho, the figures you gave from the Sierra book, (I should look at mine I guess), are diff. than the ones the software is using. For their 85 sp it shows .315, .307, .304, and those are what the program is using to do it's calculations. When Speer's .365 is used across the board, I would magine it will add up to a relatively significant diff. in trajectory, at least on the charts.
Actually, the bullet I was questioning was the Speer and the 85hpbt was just the one I picked first to compare. I must say, I probably assumed that it was a MK and was surprised that the bc listed was so low, but if it is a PH or GK it would be more believable to me.
El Deguello, that above assumption was what made me think the numbers were screwy. Comparing a MK and the Speer, one would think that the bc's would be reversed. I agree that a larger hp would change the figures significantly. It seems kind of funny that even Sierra's book gives what appears to be an artificially high number when compared to their software. I wonder why the numbers differ so much? Like I said before, I wish I had the time and money to do the shooting neccessary to get the actual results on each of the bullets in question. That would be even more interesting than this discussion. Thanks to all, Jeff.

JeffP40,

I am taking these numbers from the Sierra 3'rd edition. I have the Sierra #5 manual, but if it even shows the BC of their bullets, I didn't see it.

The Sierra #3 manual shows a B C of .329 @ > 2800 fps, .321 @ <2800 & >1800 fps, and .315 @ < 1800 fps for the 85 gr spitzer flat base bullet.

These numbers may be a little diferent than shown in your software for one of several reasons. Sierra may have revised their numbers because the bullet has been altered since my manual was printed, or they may have measured the BC using newer or more modern equipment.

When considering BC's and evaluating bullets for hunting purposes, one might as well toss out the third decimal place.

For one thing, as has been suggested on this thread, it seems that the determination of balistic coefficient may be more art than science.

Idaho, I think very much an art. Very heavy on the extrapolation. Actually, I think there are way too many variables to allow an easily attainable figure. Jeff.

Yes Jeff40, if you are trying to do it with math calculations.

But today most commercial bullet manufacturers determine ballistic coefficients by measuring muzzle velocity and time of flight over a known range or measuring the velocity at two points over a measured range. With the Oehler PBL43, once you plug in the standard atmospheric conditions, the rest is just squeezing the trigger and read the results off the laptop.

Here is an overview of BC from load-from-a-disk: http://www.loadammo.com/Topics/September01.htm