I was doing some long range work with my 6.5-08AI and noticed that my observed drop was always less optimistic when compared to what was predicted with a ballistics program.

I tweaked the Balistic Coefficient until prediction matched observation. My "Actual G1 BC" was .565, and the Lapua states it as .615.

Can anyone else verify this.

--Rootshot

While I don't have the specific number, that seems resonable based on some trends I noticed when comparing bullets. Berger, for example, just reworked their BC's to reflect real world trends so my 6.5mm 130 VLD that was listed at 0.595 now is sporting 0.552. A lower number but one that is intended to be more honest and accurate.

As a side note this thread:

http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=966431#Post966431

discusses the same phenomonom with the 30 cal. 155 weight class of bullets though the principles of reported BC vs real world BC should still apply.

Rootshot,
Your .565 sounds right on the money.
I tested two different lots of the 6.5mm 139 grain Scenars. One lot had much larger than normal tip diameters (they were 0.076" wide) and had an average BC of 0.557. The other ones had more nominal tips and had an average BC of 0.577. Lapuas radar data for this bullet is more in alignment with the 0.577 BC. I don't know why they (Lapua) don't update their advertised BC's based on the Radar data they have.
-Bryan

BC is not a clearcut, absolute number, but varies with several influences that are not part of the bullet.

Velocity is one of these, casuing BC to vary during flight, and dynamic (weathervaning) stability is another.

If static (gyroscopic) stability is excessive, the bullet will not weathervane as readily, presenting a pitch/yaw aspect as the trajectory curve increases. This affects overall drag, and thereby also alters effective BC.

Greg

I have experimented with lighter bullets at extreme distances, using rifling twists which were optimized for heavier bullets.

The combination results in a bullet that's flying with more than maginally excessive static stability. Bullet strikes were observed to have as much as 15-20MOA more drop than computational values, and visibly elongated (but not perpendicular).

The accuracy was nearly the same as the bullets which were more properly suited to the rifling twist, but drop was indicative of a significantly altered (decreased) effective BC, and the bullets were clearly not weathervaning sufficiently to strike nose-on.

Greg

Greg,
If a bullet is properly stabilized (more on this in a moment) and you reference the BC to the proper standard (G7), The BC of a bullet is actually quite clear cut and absolute.

The fact that BC changes with velocity is not so much a matter of the bullets quality of flight as it is a matter of referencing the BC to a non-representative (G1) standard. The misconception that bullets fly better at high speed is an illusion caused by the mismatch between the bullets drag profile and that of the flat based, blunt nosed G1 standard that BC is typically referenced to. If you reference the BC to a proper standard like G7, the BC will not change with velocity.

A bullet that's properly stabilized will in fact weather-vane with the trajectory quite well. In fact, that's the definition of stability; the ability of the bullet to align it's nose with the oncoming air flow. In the case of a spin stabilized bullet that has to bend it's axis with the trajectory, the bullet reacts with a series of precession cycles that effectively cause the bullet to point it's nose to the right slightly. This is known as the yaw of repose, and is the mechanism that's responsible for spin drift. Excessive stability will not cause the bullet to fly nose high on the downrange leg of its trajectory, but will make it fly more nose right, and have more spin drift. Even for cases of highly excessive stability and yaw of repose, there is no appreciable drag added to the bullet because the angle is so small.

Adequately stabilized bullets will fly with effectively zero yaw as long as the bullet is flying at supersonic speed, and doesn't have its stability challenged by the aggressive transients present in the transonic flight regime. At transonic speeds (beginning at ~1300 fps and below) all bets are off, and I agree that bullets can exhibit very different flight characteristics depending on the details of their spin rate, air density, etc. The bullet might truck right thru, might exhibit some limited amount of pitching and yawing which increases drag but doesn't kill accuracy (this seems to be the case for the example you gave), or the bullet might catastrophically tumble which obviously spikes drag, but at that point it's irrelevant because accuracy is gone too.

So as long as a bullet is supersonic and has at least enough stability (recommended twist from the bullet maker), it can be expected to fly with equal drag and BC from any rifle and have a very predictable trajectory. When the bullet slows to transonic, things get a little more sporty and unpredictable.

-Bryan

Bryan;

Well; I have been pretty much at wit's end to explain those observed effects. My description is primarily based on SWAG conjecture, since I had little more solid thinking upon which to base my conclusions. My explanation satisfied me, but that's actually quite immaterial.

Your explanation makes at least as much sense; and given the disparity in our backgrounds, all I can do is express my gratitude to you for making the obscure more clear to me.

Greg

Fabulous discussion. Thank you for the replies.

Bryan: As part of your duties at Berger, will you be publishing G7 BCs for Berger bullets?

--Rootshot

Sounds to me like G7 needs to be the standard, and G1 needs to go away.

this was one of the most useful threads I have read in quite a while thank you for all the information.

Rootshot,
It is our long term strategic objective to establish G7 referenced BCs as the standard for long range bullets. If this shift is going to be successful on a large scale, there are many pieces that have to be in place and we're working on them. I think that by sometime this summer you may see G7 BC's on our box labels and website.
In the mean time, I can provide G7 BC's for any of our bullets (and any other bullet I have data for) on request.
Take care,
-Bryan

Can anyone list the ballisticsts programs that are G7 compatible?

JBM Ballistics (http://www.eskimo.com/~jbm/calculations/traj/traj.html) is free and supports G7. I see little reason to use any other PC based ballistics program. I have compared it to several others and found that it's predictions track very closely.

I rely on a 5.11 HRT watch as a ballistics calculator when I'm out in the field and have given up on printouts. The watch is phenomenal but only supports G1. If they start supporting G7, I might be tempted to "upgrade"

--Rootshot

Great thread. How do you guys feel about the accuracy of the different BC's in Bryan Litz's book?

Given the work Bryan has put into this BC sampling project -his background , passion for ballistics , shooting ability Id say I sleep real comfortably the night before a shoot / his book applied ballistics is the new bible for shooters

I see no point to use any G -based drag models (which are all based in some standard bullet) with Lapua bullets since there is free radar -based software available in Lapua pages.
Quick Target Unlimited (Lapua Edition) uses bullet specific Cd -model which is measured with bullet radar.
Program also calculates Coriolis effect.
We have tried it several times way beyond subsonic point, and accuracy is spot on.

Some earlier chat about issue 1 2

http://www.lapua.com/index.php?id=1387

Secondly,
Air layers temp variation can cause easily 5-15" POI height differences in long ranges even if you have 100% correct drop chart. Phenomenon is called refraction.

Its good to know this- if ground is warm and air cold (or vice versa), aiming point will not be in middle of target but high or low. Bigger temp differences, bigger shifts.
it is easy to see: before warm day in early morning, place scope in rock-solid mount and lock it in middle of target, say, 1000yds away. Then compare locked spotting point at target in afternoon and evening, just after sunset. Be careful not to touch in scope.
Keep target and scope low, say lower than 1yds from ground, refraction is usually biggest near ground.

Well, some of us sometimes have need of a ballistic program when we are somewhere without a computer connected to the Internet.

Sure, but I was replying to original question. There wasnt mention about mobile software.

othervise it should not matter too much since printer paper is cheap.

Only thing missing from Lapua Quick Target Unlimited is spin drift. I use different program for that.

Thanks bryan:

Does the quoted paragraph imply that the amount of spin drift is dependent on the twist rate of the barrel?

I've shot about 20 of the bullets out to 600yd, a couple of them to 1000yd, and the new 7mm 180 Hybrids (with Bryan's supplied BC) to 1680yd.

They were all spot on. I'd be surprised to hear that he published data which he was not confident was correct. The only time I've heard it happened he published a retraction and corrected BC as soon as possible. (Ref. 300 Hybrid 338's that just came out)