True Ballistic coefficient

Shooterjoe · Aug 18, 2024

OK...I have to get this off my mind...So, Please bear with me...Dont know if I am looking to deep into this or if it's been mentioned before... A Sierra Gold Medal Match King .308 has a ballistic coefficient of .462 and G7 0.224... When I plug in these values in a computation in an app, it is correct...at this time...I dont debate this at all...My problem is the declared values of BC are the bullets in the box. So, my understanding is the barrel bore has grooves and lands in it. I know the bullet has to spin...I get that...After the bullet leaves the end of the barrel it's BC is totally wrecked by the lands/grooves marks on the bullets outer casing...This has got to screw with the computations...Is this variable taken into account by the app or formulas of the day. I dont know how many lands are inside a barrel and if they are consistent in all barrels. If it is different then would it complicate things even more?

375fan · Aug 18, 2024

There have been many discussions as to to the validity of the bc on the box of bullets. Lots of different viewpoints and experiences with said bc. From my personal experience most are pretty close. Also, you must have accurate fps along with it, fps can change based on conditions, ie big temperature swings.
You need to keep an accurate record of shooting conditions, ie a data book.
I'll try and keep simple. I do record and keep data on all rifles. Say in July when temp is hitting 90, record dialed up for say 1200 yards .
In January when it's 20 degrees, the up is slightly different due to change in environmental conditions. Bullet/BC hasn't changed but conditions have. Log/data books are your friend.

Doom · Aug 19, 2024

Actually the BC is not a strong function of the surface condition of the bullet when traveling at supersonic velocity. At Mach Numbers greater than 1.0 wave drag becomes the dominant drag due to shock waves and the viscous drag represents less than 20% of the total drag.

vh20 · Aug 19, 2024

Doom said:
Actually the BC is not a strong function of the surface condition of the bullet when traveling at supersonic velocity. At Mach Numbers greater than 1.0 wave drag becomes the dominant drag due to shock waves and the viscous drag represents less than 20% of the total drag.

^ Thus, the numbers on the box are generally in the ballpark.

To get more in detail, the BC also changes with velocity as the bullet travels longer distances downrange, and Custom Drag Models (CDM) derived from long range Doppler studies like AB has in their bullet library will get you closer than the box number at long range (sometimes dead-on, but almost always a little better than a single G1 or G7 at longer distances).

But, different barrels (even identical ones) can affect the surface differently and this can also have a small effect as well (this is not in conflict with what @Doom said above, it's a small effect, not a dominant one). If you're using a CDM for your bullet, these effects from your particular barrel can be accounted for by using the Cal MV function in AB. You put in an ACCURATE MV from a quality Doppler chrono (Garmin, Labradar, etc) , or a Magnetospeed, and shoot the distance (within 10% but never farther) prescribed by the Kestrel AB in the Cal MV section. If your elevation is dead-on, no calibration is needed. If your barrel is affecting the BC, it can be accounted for by adjusting the Drop from the calculated value to the one you observe. The calibration may be up or down, depending on how your barrel affects the bullet as it goes down the bore. Then you have a tuned program for your specific bullet and barrel that's very reliable at all distances up to deep transonic. Once you get into the subsonic distances, there's another function in AB to account for BC in that region (Drop Scale Factor - DSF, and it is performed in a similar fashion.

bulletbluesky · Aug 19, 2024

For ELR work BC is outdated concept, and in many cases introduces unacceptable error

ma smith · Aug 19, 2024

There's no such thing as a "true" BC, so the whole exercise needs a re-think.

A "trued" BC is just a correlation estimator that has gone through a "tru-ing" process. The tru-ing process is has nothing to due with "truth", and everything to due with comparing to a "known reference" (eg a "true bar").

In the case of dope, the known reference is a target downrange with your actual ammo/gun/shooter mechanics on that particular day/range/distance and environmentals etc. We are all missing that kind of grounding.

The ballistic solver models the BC "works with" are just "best guess" of flight path, and we use "stalking horse" estimators (G1 and G7) for a generic projectile. The BC is just a % of the stalking horse that makes our numbers line up.

We're a million miles away from anything being "real" or "true" whenever we are talking about "true" BCs...Not trying to be pedantic, but to ground the conversation to what is going on mechanically.

We true BCs in the first place to go from wild guesses to better guesses, but everything is still fundamentally guesses...There's never an end to the data needing to collected and revised, IMHO

ETA - for brevity

Shooterjoe · Aug 19, 2024

ma smith said:
There's no such thing as a "true" BC, so the whole exercise needs a re-think.

A "trued" BC is just a correlation estimator that has gone through a "trueing" process. The trueing process is has nothing to due with "truth", and everything to due with comparing to a "known reference" (eg a "true bar").

In the case of dope, the known reference is a target downrange with your actual ammo/gun/shooter mechanics on that particular day/range/distance and environmentals etc.

This whole discussion really needs a re-think, since its all missing that kind of grounding.

The ballistic solver models the BC "works with" are just "best guess" of flight path, and they use "stalking horse" estimators (G1 and G7) for a generic projectile. The BC is just a % of the stalking horse that makes your numbers line up.

You're a million miles away from anything beings "real" or "true" whenever you are talking about "true" BCs...

Not trying to be pedantic, but rather just to ground the conversation in what is going on mechanically.

You true BCs in the first place to go from wild guesses to better guesses, but everything is still fundamentally guesses...

there's never an end to the data needing to collected and revised, IMHO

Wow...I am out of my league here....So many variables and data for this long distance shooting. Awkward how I find this endeavor interesting, challenging and so damn frustrating...I had to look up pedantic, interesting word...

vh20 · Aug 19, 2024

Shooterjoe said:
Wow...I am out of my league here....So many variables and data for this long distance shooting. Awkward how I find this endeavor interesting, challenging and so damn frustrating...I had to look up pedantic, interesting word...

Don’t let it overwhelm you. Start with the basics, see how far they’ll take you (with a capable cartridge, 1000 is simple). When you work your way out beyond that and start to run into issues, that’s when you start worrying about the minutia being brought up here.

fmsniper · Sep 5, 2024

Try running Eagle ballistics (prs) subscription
You can tru BCs in Super Trans and Sub for a better result.

Sandow the Heretic · Sep 6, 2024

Shooterjoe said:
When I plug in these values in a computation in an app, it is correct...at this time...I dont debate this at all...My problem is the declared values of BC are the bullets in the box.

There is no true BC. The BC of a bullet changes based upon speed and atmospheric conditions. Some manufacturers will give you the BC at three velocities for this reason. This is why most of us are using models that are based upon how the BC changes over time. For instance, CDMs from AB are composites of a bullet's drag profile from different atmospheric conditions out of different guns and are going to give you a better answer than any single BC.

-Alex

jasent · Sep 13, 2024

Cdm’s can get you surprisingly close but D.o.p.e. Is better. Like said keep notes, you’ll never remember the details

Sandow the Heretic · Sep 13, 2024

jasent said:
Cdm’s can get you surprisingly close but D.o.p.e. Is better. Like said keep notes, you’ll never remember the details

Taking notes is always a good idea. Particularly since you can see patterns of predicted vs observed. Enough dope to cover most DA's and distances sounds like it would be near the life of a barrel though. I see two guys using excel to do their calculations and they are some of the few guys that can run statistical models and actually understand the caveats. Can't say I'd recommend trying to run on dope alone. Only time I've seem that done the guy hit one target on his 11th shot at it and got zero points for it.

-Alex

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