Real World BC

[Alan]

Hard Air Magazine has been the leader in providing pellet and slug ballistic coefficients for the masses (Ballistic Coefficients). The task has been rather arduous and continues to this day. Referenced in the link above, is this paragraph:

HAM Ballistic Coefficients are calculated using the GA drag model for pellets. For slugs we use the G1 model.

While these drag models certainly give us a comparison-reference between our preferred projectiles, the actual drag models are not (perfectly) matched, especially so for slugs. For example, The G1 BC profile for an NSA .250 Cal., 26.8 Gr., slug, is listed as 0.088, which doesn’t match real-world drop figures regardless of the velocity and ambient conditions. Since I use these exact slugs and own a FX Ballistic Chronograph, I can compare the POA vs. POI using the G1 BC model against real-world figures. In short, they’re exactly that, short!

A much closer match (POA vs. POI) is the RA4 drag model now available in the FXBC via firmware update. Using the RA4 drag model, the comparison is very close to real world figures, but not exactly. After ≈200 yards, the error between POA vs. POI starts to fall short as well.

For the record, I live in Roswell, NM, at ≈3,500 altitude with widely-varying ambient conditions. Actual BCs calculated by the FXBC (RA4 drag model) have varied between 0.068 and 0.077 over the last twelve months. I should add, that I use the FXBC App on my iPhone to calculate the drop for any specific shot. The BC used, is the one the FXBC gives me. The ambient used is what it is.

I’ve never been a scope knob turner, but I have learned doing so improves the hit rate. This said, most of my pesting is done under 125 yards, using a BC of 0.072 (FXBC average measured RA4 BC) and holding over. Doing so, the POI is very close to my 87 yard, Point Blank range calculated (range card) distance.

I’ll agree that $900 is a lot of money to spend for a calculating ballistic chronograph, some folks view as a toy. I can assure you, it is not!

 

[Arzrover]

Keep in mind that shooting into or with the wind can substantially affect your calculations...
Even a 45 degree crosswind will affect them.
I've measured a LOT with the testing of so many barrels and typically TRY to throw in some previously measured combinations for comparison. I've never tested anything as low as the HAM charts show. I'm at 3000 ft elevation. Ballistic calculators tend to confirm my measurements pretty consistently.
Been using a LabRadar for almost all of it.
Bob

 

[Alan]

Yes, but what are you shooting (slugs, pellets?), and what BC profile are you using?

Just for the record, I do have a rather long hay barn to shoot inside of, until the harvest fills it up! This gives me a relatively low wind scenario. As I stated, I'm at ≈3,500 feet, and the ambient often gets to 100+°F. Lows can hover down to the teens, and sometimes into the single digits. I stay in during the worse ambients, but during winter, BCs often go much lower than one would expect.

 

[Arzrover]

I've used G1 and GA in Chairgun, Strelok, and EasyBC. Not seen much difference till I'm over 100 yds. I've shot a literal ton of pellets and a lot of 177 and 20 cal slugs with much less 22, 25, and 30 slugs.
In the interest of staying comparable, HAM uses a less than ideal setup but it's likely repeatable... In my own testing, many times over multiple sessions, the FX smoothtwist and smoothtwist X gave lower bc calculations than the LW poly's I was testing. The standard rifled of various brands were less than the FX. The slugs were DIFFICULT in the poly's so I don't have a lot of data there. Also, I've mainly shot NSA slugs and all my results were higher than they specify. In talking to Nick about that, he said they are conservative estimates based on his own and a couple of friends testing... so no really careful, in depth analysis. In other words... your results will probably be different...

If you've found a good solution for your uses, why worry about others... just use what works...
Bob

 

[Miles_M]

If you have a LabRadar it can be used to produce purpose drag laws for each of your pellets which is a much more accurate system than BCs which will always have fundamental flaws. However, it all depends on the level of accuracy wanted. At many airgun ranges, using a BC will give an accurate enough answer, but as speeds and ranges increase then the purpose drag laws are needed.

In my opinion, Doppler radar systems are pointless unless they can print out detailed times and velocities over a significant part of the trajectory. By that, I mean a time and velocity every few milliseconds, not just every 5 or 10 yards. For muzzle velocities, there are plenty of alternative methods for measuring them which will be fundamentally more accurate. A Doppler is the only instrument which can give detailed data throughout the trajectory, but not at ranges below about 10 yards from the muzzle. Using it to give just a muzzle velocity is, in my opinion, a waste of the technology. They can also be tricky to set up accurately.

Any method used to obtain BC values or purpose drag laws can only be accurate if the atmospheric conditions at the time of the test are measured or obtained in some other way, and used in the calculations. If the atmospheric conditions are not known or used, the results you get will only be applicable on days with those exact same conditions and comparisons with other results will be meaningless as they will always be different. Even if you do obtain your BC or purpose drag law with the true atmospheric conditions taken into account, you will still need to feed in the atmospheric conditions into your trajectory apps at the time you are using them to shoot to accurately predict the trajectory on any different occasion. It is not as straight forward as many think and, as I said above, it all depends on the degree of accuracy wanted.

 

[Alan]

I agree that most BC figures can be misleading with respect to distance. However, knowing what the calculated BC is over a long distance, then using it to determine the drop data at short distances, doesn't make it (drop data) any more accurate.

What's more, we're not the military, and we're not determining the drop data of a naval rifle out to 18 miles or so. Fact is, most pellet airguns are limited to less than 200 yards. I doubt seriously, if measuring the drop data even foot would be any more accurate than measuring drop data every 25 yards or so.

The slugs I shoot have been tuned to 1,011 FPS. For practical purposes, that means the slug travels 1 foot each millisecond. Thus your statement (I mean a time and velocity every few milliseconds, not just every 5 or 10 yards) is a bit misleading.