Calculating BC

[aubrey]

Hi all

Great forum, thanks! What's the best way to calculate the actual BC with live firing, chronograph etc?

I'm also experimenting with bullet pointing, how best to calculate the BC improvement from a known BC like 155.5 to "actual"?

Thanks!

 

[Joe Grad]

A simple but less accurate way is to just measure bullet drop at different ranges. You should be able to estimate BC directly, assuming you can find the center of your group. You can also use a pair of chronographs. Bryan Litz measures his BC's with a chronograph/audio microphone setup described in the book. The Applied Ballistics book talks you through each of these.

http://appliedballisticsllc.com/

I'm not sure how accurate you want to be but you can also estimate the change in BC by using the i7 form factor regression equation in the same book. If you can measure the change in meplat diameter, the regression can approximate the decrease in G7 form factor and the corresponding increase in BC.

-Joe

 

[CatShooter]

Some of the better ballistic software can come very close if you plug in all of the dimensions.

Some people think measuring muzzle velocity and long range drop is not accurate... but since it is drop that we are after, so in a sense, that is the ONLY measurement that really counts.

I use RSI and it will fry your brain with information.

 

[Bryan Litz Ballistics]

Hi all

Great forum, thanks! What's the best way to calculate the actual BC with live firing, chronograph etc?

I'm also experimenting with bullet pointing, how best to calculate the BC improvement from a known BC like 155.5 to "actual"?

Thanks!

Measuring BC from drop is the most error prone way to do it. Consider that, if you're looking at drop from 100 to 600 yards, a 2" error in your drop measurement will result in a 10% error in calculated BC.

Measuring velocity loss between two chronographs is the next best way, but this technique is limited to the range you can reliably shoot thru the windows. Also, you have to sync your chronographs to insure they're reading the same.

Measuring MV and time of flight is more practical for long range because you don't have to go thru a window (just shoot within 10 feet or so of a mic. However the equipment for doing this is not commercially available.

It's quite difficult to make accurate measurements of BC over long range. Getting within +/-1% is deceptively difficult, considering all the things you have to measure, and measure accurately.

Good news for you, if you're looking at a *change in BC* due to pointing, then you don't have to be accurate to 1%, you just have to be able to make good 'comparative' measurements.

So lets say you set up two chronographs, 200 yards apart. Not normally a long enough distance, but you can easily keep the shots in the windows. Now you shoot a bullet, get the beginning and end velocity, and you calculate BC of .300. Now that might be 10% high or low of *actual*, nevermind that. Now shoot another bullet, perhaps the pointed version of the same bullet, and you get a BC of .315. You can't say that you know the actual BC's of the bullets you fired with great confidence, but you *can say*, that the second bullet is 5% higher than the first with pretty good confidence. (0.315/0.300 = 1.05).

Think if it like a comparative measurement. You typically don't care what the base to ogive measurement of a bullet is, but you do care how much they vary. A bullet comparator allows you to measure relative things.

The BC test described above is just such a comparative measurement.

Again, it's very difficult to measure BC within 1% accuracy, but by making comparative measurements at the same time, you can more accurately figure the difference. Note, in such tests, you'll want to shoot the pointed and unpointed versions back-to-back, in the same session, so that uncertainties of various test set ups and conditions don't creep in.

This method is also possible when looking at drop at long range. Shoot a group of unpointed and pointed bullets at long range (preferably round robin so barrel heating/fouling effects are mitigated) and note the difference in vertical group centers. Again, although a comparative measurement is more valid than a single measure of BC from drop, I would still favor the velocity loss method over the drop method based on error budget. You might be able to resolve a difference in BC from drop well enough in a rifle that's capable of shooting 1/4 MOA groups at 1000 yards, but there's just too much uncertainty in the group center for most rifles at long range to be sure of the group center.

To catshooters point; yes, drop is what we're ultimately interested in when doing trajectory predictions including wind deflection. However, sometimes the most direct effect is not the highest resolution method available for measuring.

-Bryan

 

[r bose]

"Measuring BC from drop is the most error prone way to do it. Consider that, if you're looking at drop from 100 to 600 yards, a 2" error in your drop measurement will result in a 10% error in calculated BC". copied from above

Amazing what I've learned reading from this site. I find most shooters are so "BC occupied" they buy, purchase and shoot bullets based on just this one factor. So, to say it backwards, a projectile with a BC 10% lower than another projectile will only impact the target 600 yards away 2" lower? If of course impact is the same at 100 yards?

I'm guessing the full value wind drift difference is very small also?

 

[Bryan Litz Ballistics]

At 600 yards, the difference in wind deflection, for a 10 mph crosswind, is about 3" for 10% difference in BC.

Of course, these are 'rule of thumb' generalizations. Exact amounts vary based on MV, atmospherics, etc.

The difference is greater for 1000 yards. At that distance, for a 10% change in BC, there is a:

19" difference in drop, and
11" difference in wind deflection for a 10 mph crosswind.

These may sound like small amounts, but for those competing in the high levels of F-class and BR, these effects of 10% BC are significant.

-Bryan

 

[aubrey]

Thanks Brian, very useful insights, much appreciated!

 

[Laurie]

At 600 yards, the difference in wind deflection, for a 10 mph crosswind, is about 3" for 10% difference in BC.
[Bryan Litz]

I always ask for my outputs as MOA in a 10 mph wind as it's easy to calculate the effects on a per 1 mph basis. ie 10-MOA drift at 10 mph = 1-MOA / mph. Go to the range and it's blowing 8 mph from a quarter direction and = 8-MOA divided by 2 = 4-MOA on the scope for the first sighter (taking off or adding a bit at long-ranges for right spin-drift too).

And ... 10 mph values are nice and easy to compare and almost everybody quotes them on forums etc. BUT .... once one is on the target having hopefully got the initial wind adjustment somewhere near correct, it is shot to shot changes in wind strength and direction that matter, and 10 mph is a vast value.

A benefit of the UK and British Commonwealth form of TR, MR, and F where we shoot two (often three in club matches) to the mound is that we keep full target shot-plots (or should do) including both estimated and actual windage. eg reckon you need 3-MOA right on looking at the flags and other indicators, score a 4 just over the line at 3 o'clock and the actual windage entered on the plot sheet is 2 1/2 right. Looking at series of plots for matches at different ranges and using Bryan's PM 2 program allowed me convert real wind changes in MOA back to equivalent 90-deg wind speed changes I say 'equivalent' because the wind on the day might have been blowing from 5 1/2 o'clock, 10 o'clock or whatever. I've rarely seen changes between shots that exceeded 3 mph on such a basis, and shot whole matches in difficult conditions that didn't see a single shot to shot change that was above half that. Over the match as a whole, there is often a much larger range of equivalent speeds, but that's irrelevant.

Remember that shooting two to a mound, that we have three or more minutes between shots. You guys and girls in the US and 'down under' who string shoot will likely reduce that gap dramatically which reduces the chance of large changes between any two shots considerably. If you see a big change, you stop and wait - we don't have that option (the 45 second rule!).

Anyway (belatedly!) the point about this is that ballistic comparisons between models using the 'standard 10 mph' wind speed often massively exagerrate the performance differences between models. People tell you that bullet A moves 5 inches more sideways than bullet B at 1,000 yards and you picture the target centre in your mind and think Wow!, that's huge! But, that's a little under 1/2-MOA and at 10 mph. Actually it's 5.0 divided by 10.47 = 0.48-MOA or 0.048-MOA per 1 mph, half an inch actual difference between the two designs.

Let's add in the overall effects alongside comparing the two models. Let's say it's the 155.5gn .30 Berger BT Fullbore at 3,050 fps. The ballistics program says the 10 mph 90-deg crosswind moves the bullet 8.51-MOA at 1,000 yards under standard ballistic conditions, ie 0.851-MOA / 1 mph change. The other 'better' bullet that moves 5 inches less in the 10 mpoh wind, sees 0.851 - 0.048 = 0.803 / 1 mph movement.

The real wind speed change in 90-deg terms was say 3mph and you read that approx 50% right adding 5 clicks windage to a quarter-MOA adjusted scope for 1.25-MOA when just over 2 1/2 MOA was needed (0.851 x 3 = 2.55-MOA). This was a poor call in a relatively big change and assuming elevation, zero, and aim were perfect and the shot would have hit the group centre on a perfect wind call, you certainly drop two points and are pretty close to dropping three - you will if the elevation is significantly above or below the central 'waterline', or the bullet strike is at the outside of the group dispersion such that it reinforces the wind drift. If none of these things apply, you're 1.276-MOA awry.

What about the 'better' bullet. Same 1.25 mph wind under-read error, now becomes 1.205 MOA 'out', 0.071-MOA or a shade under three quarters of an inch - same score obtained assuming the same factors (perfect elevation etc) apply. At 500 yards, same difference in MOA but a mere third of an inch on the paper.

BUT (again!) what if the lower BC bullet holds its elevation better, and/or shoots tighter groups at 1,000 yards? A relatively small change here can easily overcome the apparent ballistic disadvantage, and in fact give a higher score.

This is not to argue against people and companies producing higher BC bullets, just to say that (a) it's not everything and that (b) the commonly quoted 10 mph 90-deg wind comparisons may (wildly) overstate performance differences, and that (c) other factors may have as great as, or greater, actual effect in real life match conditions.

As r bose says in an earlier post

I find most shooters are so "BC occupied" they buy, purchase and shoot bullets based on just this one factor.

 

[r bose]

Laurie, this was my point exactly but wasn't sure how to put it into words as you have so well.

Recently I was shooting the 82g berger's in my 223 bolt gun at 600 yds. Brian Litz, in one of his post suggested I try the 80.5g bullets and they shoot so much better at 200 yards in my gun. (at my test range). But, the first thing a friend said to me was "why are you willing to give up the BC points to use the 80.5g bullets".

The answer is, A very small reduction in BC is worth it if I can shoot a bullet that groups almost 50% tighter than the other higher BC bullet.

Thanks to all.