I mentioned this before in the humidity thread: BC's are said to be "system sensitive".
The BC's that are mentioned in the bulletmaker's literature may be close to a "real world" value (like Sierra likes to point out), or a higly optimistic value only achievable under near perfect conditions. The BC of a load depends not only on the qualities of the bullet used (absence of flaws, form factor, weight, uniformity and concentricity, bullet construction, etc.) and muzzle velocity, but also on all the other components of the load: case, primer and powder, and how they are assembled can influence the initial yaw of the bullet. The BC during the first 100-200 yards can be heavily influenced by this coning motion, and BC is probably more predictable past this range. Barrel twist, rigidity and harmonics also influence this initial yaw, as does the muzzle crown. Besides the barrel lenght and contour obviously influencing the harmonics (as does the action,stock, bedding and everything else in the rifle), longer barrels are less rigid (larger deflections at the muzzle in the moment of launch, if you are off the "sweet spot"), but also have lower muzzle pressure to destabilize the bullet.
I don't think the operator can influence BC, but the zero often changes from person to person (why? ... I don't know, maybe different ways of handling the rifle and different recoil).
Can all the above factors be the reason why some 1:12 barrels shoot 175gr. HPBT bullets fine and some not? If your initial coning motion is relatively large, it will damp out past 200 yards, but this higher initial drag will also influence flight dowrange (a bit less retained velocity).
Hope to hear your comments and corrections.

Ingalls defined the Ballistic Coefficient of a bullet as its ability to overcome air resistance in flight indexed to Krupp's standard reference projectile.

Essentially, BC defines the ratio of velocity erosion due to air drag for a particular bullet.

Yes, the BC is defined as the ratio of the drag deceleration of the standard bullet (ussualy taken from the "G1" tables based on Krupp's bullet) to the drag of the actual bullet.
According to Sierra's manuals, the BC is highly dependant on coning motion, and this is why they recommend measuring the BC at least 100 yards from the muzzle, when the initial coning motion damps out (and many people swear that some bullets take a lot longer to stabilize). It is a known fact that BC's for the same bullet are a bit different when fired in nearly identical rifles; and some of these rifles will stabilize some bullets better than the others, and I just want to know why.
The only factor I can think of to explain this anomaly is the difference in the bullet's initial coning motion out of the various rifles, and my initial post was to discuss the various factors that influence it.
I hope some of you can further educate me on this topic.

It's just hard to tell where you are asking questions, quoting Sierra, and making statements in your first post so I wasn't sure exactly what you are/were after. Then again I might just be brain dead today.

I think you are looking at BC the wrong way and or reading too much into it. Plus, there are different theory's on how to measure a bullets "effectiveness in flight." On one hand you are talking about barrel harmonics (we have a VERY GOOD article on this and it will be posted soon)and on the other you are talking BC. When you sit down to shoot, you don't figure the BC of every shot. There is no need to do this. You find a bullet with a high BC to help reduce other factors and get better accuracy at the target end. I strongly believe that consistency is more important than a BC in our type of shooting.

Now if you want to look at what changes BC then look at what changes velocity. Primer, powder, case, and bullet will of course all play a part in that as will meteorological conditions. Barrel and related conditions will also change velocity but I don't think you can say a stock, bedding, trigger, etc. play any part in that.

So what I see as your main question is why do some guns shoot particular rounds better than others (in identical rifles). Here I think the harmonics play a crucial part as well as the actual barrel. We can line up 10 barrels from the same company off the same "line" and all will shoot a little different. Just look at what goes into each one. Each barrel is treated a little different and has a slightly different "make up" just like a finger print. Then you have other factors like the chamber, crown, as well as high and low spots in the barrel. Again, consistency is the key.

Tiro, Let me throw some gas on the thread. You pointed out that the same bullet may or may not stabilize from barrels of identical twist rate with identical loads. No problem.

Some barrels are much better than others. If the bore is straight as in nicely cocentric to the centerline of the barrel, if the bore is of uniform diameter for both the lands and the grooves, if the bore is smoothly finnished, if the twist of the rifling is uniform throughout, if the chamber is cocentric with the centerline of the barrel, then the bullet will be relatively stable in its motion through the bore. If it is relatively stable upon exit, that is the crown does not contribute to the upset of the bullet excessively, then a marginal twist may stabilize the bullet and provide pretty good accuracy. Remember that midrange tip over contributes to long range accuracy.

If the barrel is less than optimum, then the bullet may NOT be stable enough upon exiting the barrel to fully stabilize in flight with the same twist rate.

If a bullet is coning or precessing, it is not presenting the optimum aerodynamic profile which wastes energy, which in this case shows up as a loss of velocity. Since it looses velocity because of less than optimum stability it will compute as the equivalent of lower BC since BC relates to the rate of loss of velocity. The EFFECTIVE BC is less with an unstable projectile.

No big secret really. A better barrel shots better.

One other point that bears mentioning is that BC varies with velocity. Increase velocity, increase the BC. Up to a point, but then no one is using a electromagnetic rail gun to hit that particular point either.



[This message has been edited by Rust (edited 02-01-2001).]

snip1er, sorry for my confusing post. I know all you have to do is pick the bullet that suits your needs and proceed (no real need to measure BC yourself, ever, just measure the velocity and check your real world POI at different ranges with an external ballistics program to see if there's anything funny) but I would like to get a better understanding of the theory involved.
You are right in what consistency is much more important than a little gain in BC, and I was looking at the problem that way.
If the slight imbalance of the bullet at the moment it departs the muzzle is important to BC/initial stability, then all the factors affecting the barrel harmonics can influence it, such as stock and bedding. If the bullet departs off the "sweet spot" the chances for tipping are greater.
Regarding the loads, velocity will influence BC, but not so much, with normal loads we are talking only about 150 fps difference.
Take a look at the drag models in the 2600 to 3000 fps area at: http://207.181.246.106/johns/extbal.htm
In any case, I'm more interested in the coning motion, its factors, and how it affects the bullets performance. Regarding this, I think the muzzle pressure (lower pressure, due to faster powder and/or longer barrel is theoretically better) can be more important to tip the bullet (and hence affect initial stability/BC).
Rust, thanks for your comments on barrels, it makes a lot of sense, but why do nearly perfect "super match" barrels behave so differently in benchrest competition?

Tiro, The mid range tipover I refer to is required for good accuracy. What it describes is the ability of the centerline or axis of the bullet to follow the path of trajectory in flight. A bullet that is over or understabilized does not do this and the accuracy is adversely affected.

As far as even best grade barrels not shooting the same? Think about it, are they really identical? They cannot be.

Despite the best equipment and instrumentation it is impossible to make two identical objects of any sort. Grain structure will vary, stress levels will vary, there will be a 1/100,000th here, another there, the barrel installation will not be the same, even mounted on the same reciever the interface cannot be identical, the rifleing is not perfect.

So if you cannot produce to exactly identical systems, not just the barrels, how could you expect them to perform identically. It the level of precision of shooting required for benchrest the most minute of deviations will show themselves in the aggregate, plus operator error.

Gets interesting because it is beyond the current limits of technology to produce identical items. That is why tuning the load and rifle is still required.

By the way, non uniform bullet flight, as in coning, is also more susceptible to envoronmental influence because it presents a less uniform profile.

You have to view the whole thing as a system. The firearm is a system for sending a projectile to it's intended target. Alter on thing, alter the system, alter the delivery of the projectile. The projectile is part of the system too.

About Ballistic Co-efficients.

Some of you guys may know about the data tables that I developed for several civilian and military cartridges. I make the claim that these are the most accurate data tables around and they are not generated off of any computer program. First about BCs.

I prefer to call the numbers that I work up "Flight Co-efficients". The reason is because of the method I use to calculate co-efficients and elevation settings for various rifle scope designs. The methods for shooting and calculating these values are expensive and extremely man/hour intensive.

First off, the BC that is assigned to a bullet by various bullet manufacturers is based on a number of factors: weight, boattail angle, length of bullet, bullet material, form factor, and some calculate those figures and use an average over a certain distance, like 0 - 1000 yards for example. That figure that the factory providees is usually high, but again, it depends on some of the factors guys listed above, such as coning, etc. But, that initial figure always changes.

For example, the Hornady AMAX advertises a BC in the order of 1.050. Over the first 100 yards, the figure is more like .940. which is in the order of 11% lower than the factory claim. If you were able to plot the value of the BCs over 2500 meters for example for a .50 caliber Ball round, the plot would like like a sine wave with a low spike where the bullet begins trans-sonic flight.

I'll let a little SOE secret out. Here are the BCs that I use to calculate a data table for a certain .50 caliber projectile at 2840 fps.

100 meters BC = .6344
At 1400 meters (around trans-sonic range)the BC is .5636.
At 2500 meters, the BC is back around .6283.

Notice how the BC value dips it's lowest at the point which the bullet is beginning trans-sonic flight. This is because the super-sonic shock wave has opened up from it's initial position on a 30 degree angle to the projectile to it's current position of abouit 90 degrees or perpindicular to the flight path of the bullet. The bullet is now fighting it's way under the sound barrier.

What does this mean to long range or any range shooting. If you are going to use a published BC or a BC that you determined at a short range, your elevation values will be grossly off at ranges where the bullet is beginning trans-sonic flight. I use a varying BC every 100 yards or 100 meters and determine a standard decay rate for a measured BC.

Here's how relative this all is. If you use a computer generated table using the average BC of .6001 over 2500 meters vs the elevation settings that I have calculated, here are the differences of a computer generated plot vs. my method (which has been shot in a tried and true for several years now).

1000 meters (computer plot) = 36.6 MOA
1000 meters SOE method = 36.75

1500 meters (computer plot) = 74.5 MOA
1500 meters SOE method = 78.50 MOA

2000 meters (computer plot) = 134.1 MOA
2000 meters SOE method = 134.25

Look at the differences at the mid range and most important because that's the super sonic max of the bullet under standard atmospheric conditions. How do I do this? It's not easy.

I first shoot a cartridge in on a range every 100 meters and write down the elevation angle setting needed to obtain point of aim / point of impact at that range. Once this is all done, I go home (takes about 20 rounds per range past 1300 meters for a .50) and calculate the ballistic co-efficient (flight co-efficient at that point) using the Ingall's Ballistic Tables and a reverse elevation formula that I made up. Once this is done, I correct these FC/BC values to standard air conditions. At that point, I calculate the elevation settings off of each of the BC/FC values for each 100 meters. For 2500 meters, this takes about 4 hours of calculator time, just to obtain the elevation settings. I then take all of the BC/FC values and adjust them one at a time for changes in air temperature every 10 degrees and changes in air pressure every 01.00 In Hg. After massive number crunching on a HP20S I obtain correction factors that an operator either divides his elevation setting or multiplys it to correct for the air conditions.

I then calculate remaining velocity, fall angles, flight times, spin drift corrections, maximum ordinates, windage data in inches deflection, MOA corrections and MIL holdoffs. I also calculate danger space dimensions for various target sizes to use with range determination methods. Any of you who have seen the book on HTI or the data tables how has an explanation why they are so expensive. The first .50 caliber data tables that I made took nearly a year to complete. I am just now finishing the tables for the .50 Cal. AMAX round at 2700 fps. Has taken 4 days of calculator time so far, about 5 hours a day.

Whew,,, I'm out of gas.

Wow!!! Triggerfifty, do you pick a calm day for these tests? If you measure drop (by noting the elevation settings) and work backwards to get the BC, do you need a small, round group, or you just disregard the horizontal deflections?
I see you use the G1 drag model to calculate your BC's, is that because it fits best, or because it's the most used and easier to compare to the published ones?

You bet I use a calm day. I watch conditions for a couple of weeks prior to doing this and shoot during the most stable time of the day. At times, I have setup and waited for hours to get 5 shots off to confirm a calculation. That's why these tables cost so much.

As to the G1 Drag Function. Dr. McKoy has no problems using the G1 for this method. He says that since you are shooting data in, back calculating for a specific BC and the correcting to standard air conditions, the G1 works fine. Basically all you are doing is determining a relative value at a specific range to re-calculate an elevation setting.

The reason that this method doesn't work for what is commonly trained is that the data is not corrected to a standard condition and then corrected to new conditions.

As far as groups, I usually ignore horizontal dispersion. For vertical spread, I require 1.5 to 2.0 MOA at the target at any range to call it a verified group. At the extreme ranges, I will shoot a high number of shots, not just a 5 shot group and call it good. If money were no problem (as in government funded) I would shoot 50 rounds at each range to get a group of shots and take the value for the center 50% of that group of shots. Something similar that is explained in my book for determining a gun/system/teams hit probability under specified conditions.