I randomly stumbled across this article while looking for 308 150gr etip loads.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA555975

Is there anyone out there that can speak to the 150gr etip or the article in general? The reason for choosing the 150gr etip over the barnes for example was the high bc of .469. If this is ~40% over inflated as the article suggests, that's a real bummer.

Any thoughts?

It confirmed what I already knew, that ballistic programs are a starting point and real world dope must be confirmed by recording actual drops at distance for specific cartridge, bullet, rifle combo.

Never used the Etip, but a 150gr 30 cal bullet with that advertised BC is surely inflated by the the mfg., as is the case with most being on the optimistic side.

No bullet has a single, fixed ballistic coefficient. The BC quoted by a manufacturer is a weighted average - and we can assume that they "weight" it as advantageously as possible, to interest those gullible reloaders who buy bullets based primarily on their BC.

Slam Down!!!

And I couldn't agree more.

Many bullet manufacturers determine BC exactly as desribed, by using two chronographs. Others use a bullet model, but that's rarer these days.
Much depends on the range between the chronographs. If it's only 100-200 yards (typical of indoor ranges) then the BC may be higher or lower than it would be over, say, 400 yards, due to a barrel crown or rifling twist that allowed the bullet to settle quickly--or not so quickly. Of course, BC also varies due to velocity, as well as altitude, humidity, etc.

Thus the listed BC's of most manufacturers are much like New York City traffic laws, suggestions modified by real-world results.

"It's better to keep mouth shut and...." you know the rest BUT

I've never understood how BC can vary due to velocity.

Can it be explained in less than 10,000 words ?

With target bullets, generally it ended up about .020 less than what was advertised according to drops out to 600 or 1000 yards in the end.

FWIW RE the high BC of the 150.... remember, Sierra managed to make the 155 the same as the 168..... its in design of bullet, not weight of. But .500 seems high regardless...

Just a SWAG but think of a bullet moving thru air like a boat thru water.

At some speeds your boat simply pushes more water and is inefficient, get the boat moving faster and your wake gets smaller, boat's more efficient etc - moves less water.

As a bulled moves through the velocity spectrum - it's going to push more or less water - depending on shape (how efficient its 'hull' is) and how fast it's moving.

ETA - BC is simply a measurement of that 'hull efficiency' - it will change as the 'boat' changes speed as it moved through the air.

Just my uneducated guess.

The BC attempts to describe--with but a single number--the aerodynamic drag on a bullet over a great range of speeds, from the low subsonic, through the transonic, to well beyond supersonic. Like many ambitious simplifications, this one isn't always as accurate as one might need. Attempts to assign a BC at 3000 fps can result in extrapolation errors at 1500 fps. If such an error is too large, you recalibrate your equations to give a BC which is more accurate for speeds near 1500 fps.

I can provide a more complete explanation if you desire, but that's the crux of the problem.

2525 is correct. drag does vary greatly depending on velocity, and it can be a very complicated thing to calculate and most of the knowledge we have has been obtained through testing. formulas have been created using the test data to try to mimic the real world results, however with so many variables that all can have a huge impact on the results, it is virtually impossible to know for sure what is going to happen until you actually get out and see what happens.

Many problems with the study as I see it. To claim that manufactures are publishing BC's that are not true when they are starting with different parameters than what gave the manufactures their results is more misleading that the manufactures publishing the high BC's.

The 150 grain E-tip for example, may have been tested by Nosler at a starting muzzle velocity from say the 300WSM at nearly 3200fps. This would cause a big difference in the measured BC at 500 yards as the article suggests. So saying that they intentionally over hype the BC is not necessarily true.

Second the G1 profile is not a good profile to measure most modern boattail bullets. The farther the bullets vary from the form factor of 1.0 the less accurate the BC of that bullet will be over a wide range of velocities. Thus the need for the G7 BC. Unfortunately the the G1 has been used for so long that many manufactures use it because that is the number that shooters understand when looking a BC's. Giving a G7 BC on bullets and people who use BC to decide which bullet to buy will not easily draw a comparison. Because the form factor is not close to 1 with the G1 BC the bullet will drop off more rapidly the farther it get away from the velocity range that the published BC was derived from. Since most manufactures don't test and publish their BC's from velocities of in the 1500fps range it is not surprising that the BC's in the test drop so far from what is published.

Perhaps we need a Federal agency which will provide uniform standards and and official number for each of every manufacturer's bullets. And regulations prohibiting the publishing of other than government approved figures.

THNX guys -

Makes sense ENOUGH. It just didn't make any sense for the BC to change based upon its design alone.

The analagy of the boat/water made me think of gas mileage in autos. At diff speeds you can get diff mileage. Maybe an oversimplification but......

I've know for a long time that ALL BCs are NOT accurate.

THNX again.

The concept of Ballistic Coefficient can be traced back to Francis Bashforth to around 1870. His idea was to use the flight of a standard bullet as a basis for predicting the flight of other bullets. In its simplest form Ballistic Coefficient is just the ratio the aerodynamic drag of an actual bullet to that of a standard bullet. There are many standard bullets, but the two most common are the G1 and the G7, which you can see have very different shapes.



G1 is the official default standard, but Berger and Lapua are also publishing G7 BC values for some of their bullets.

So how does a BC change with velocity? The answer is that actual bullets don't exactly match the shape of the standard bullet, so at some velocities an actual bullet may have more or less aerodynamic drag than the standard bullet. We're not dealing directly with physical aerodynamic forces, but with how two bullets compare as they react to those physical aerodynamic forces.

The good news is that between Mach 1.7 to Mach 2.6 (1895 to 2902 fps), aerodynamic drag is relatively insensitive to bullet shape. This can be seen by comparing the drag profile (bullet retardation relative to velocity) of the G1 and G7 standard bullets. A curios coincidence is that in this velocity range the absolute drag of these bullets is a 2 to 1 ratio, thus a G1 BC of 1.0 closely matches a G7 BC of 0.5.



Of course some doubt this can be correct, but anyone can prove it to themselves using their favorite (but hopefully accurate) ballistics software. Run a comparison between a G1 BC of 1.000 and a G7 BC of 0.500 using the same attitude and atmospheric conditions and a MV of 2900 fps. You'll find velocity and drop are nearly identical out to 1200 yards. Even if you start with a MV of 3200 fps, the difference in drop will be just an inch or so at 1200 yards.

When you think about it you realize the accuracy advantage of using G7 BC values is insignificant for the vast majority of hunters, which is why G1 has served so well for so long.

M L - 2 thumbs UP !

I still prefer empirically verified drops with my actual round..... any program will get you 'on paper'... but you still gotta shoot it and fine tune the data. Same is true of advertised BC.... it's only there to get you close.

Better watch saying stuff like that. The engineers will show up and tell you you don't have a clue.

Of course engineers who actually know something about the subject will tell you it's a good idea to fine tune the BC value of the bullet you use, in the load you use, in the gun you use, at that distance you use as long as you test when there's no head or tail wind and you can accurately measure MV, range, attitude, temperature and the reported barometric pressure. If you don't have all that you're just fooling yourself.

Again, it always pays to shoot for real dope after figuring out where impacts "should be" by looking at a ballistics program. The accuracy of various erector systems also factors in when folks start spinning turrets and concluding that published BC's are off. I noticed a fair discrepancy the other day with my 338WM shooting Nosler 200gr BT's at a slightly downloaded 2,860fps (chronographed). According to Leupold's MOA specs and JBM's plotted impacts, my bars on my 3.5-10 B&C reticle should have be on at 300 and 400yds when my 100yd impact was somewhere near 2" high. After dialing it in at 400, and also verifying impact with the 300yd bar, I was hitting the 100yd target roughly 1" high. Go figure. Is it related to BC? I don't know, but you can bet that's how I sight in for holdover reticles, by getting the dots or stadia lines on first and then coming back to find my exact 100yd POI.

Your 2860 fps 338WM load is right at the limit for Group A cartridge for the B&C reticle, but I assume you're using the large triangle power selector. At that power there's a 2.6 MOA difference between the 300 and 400 yard bars. Thus, for a load to be on the appropriate bars at both 300 and 400 yards the load has to have 2.6 MOA difference in drop from 300 to 400 yards. Your 338WM load comes close if the Nosler 200gr BT's you're using are the CT Ballistic Silvertip with a BC of 0.507 fired at 2860 fps with an air temperature near 80 F, and they'll be 1 inch high at 100 yards as you discovered. If you check you'll find you're zeroed at around 167 yards.

Of course you would grow old and give up trying to figure that out if you're clunking around in a browser on the JBM site.

"The deer stood at a lasered 537.5 yards and the wind was blowing 14.5 knots 111� left of the target line, so I knew with my .421 BC bullet at 3113 fps zeroed at 2.16 inches high at 100, I'd have to hold 3.23 inches high and 5.68 inches left. I started counting hairs from the center of his shoulder using my 36X scope and had counted up 457 hairs and was still counting past 1238 hairs left when..




... the deer dropped over dead of old age."

What, you didn't take out your smart phone, launch a ballistics app, measure and enter the temperature, altitude, inclination and calculate a shooting solution? How did you ever expect to make the shot?

If you're like me, and I know I am, I do all the planning and research at home and testing at the range so that I don't need to mess with it in the field. If I'm not confident I can make the shot, I pass. Tracking a wounded animal is not my idea of fun. It's a different story if your hunting for survival, but luckily I've never been in that situation.

So the wind drift of a round nose is about the same as with a spitzer?

Mac, thanks.

In real life, I loaded a moderate cartridge to a bit under maximum speed with a mid-weight cup-core bullet, sighted it 2.5 inches high at 100 yards, and then just held on hair out to my self-imposed limit of 250 yards. Never needed a second shot on a deer. Simplicity.

As of late I do not pay attention to published BC`s.What happens at the range or on game is all I care about.Some of the highest BC bullets I have used were not too good on actual game or paper at reasonable shooting distances.If I plan on shooting up to 400 yds.I see what happens at 400yds. with the bullets I want to use.

The point I was making is the relative drag between bullets of different shapes is nearly constant between Mach 1.7 to Mach 2.6. At least that's the case between the G1 and G7 standard bullets as can be seen from the drag profile graph and verified with any good ballistics software.

But to answer your question, yes, wind drift of a round nose is about the same as with a spitzer for bullets that have the same BC. The different shapes, however, means the RN needs a much higher sectional density to have the same BC as the spitzer. Here are two 0.308 diameter bullets with nearly the same BC.

120gr Barnes Tipped TSX FB #30871 with a BC of 0.295
220gr Hornady RN #3090 with a BC of 0.300

If you run these in some ballistics software you'll find that they have less than an inch difference in wind drift at 500 yards with a 15 MPH cross wind. If they had the same BC there would be no difference at all. You see, BC takes into account all the factors that affect aerodynamic drag such as sectional density and shape, so any two bullets with the same BC have the same trajectory and wind drift. Where you will see differences at longer range is due to spin drift, but that's a whole other subject.

I read the publication and it is very interesting. At the end, the authors discuss the �Litz barrel whip� phenomena and its possible effect on BC. This theory attributes lower BC to the effect of light weight barrels whipping and the bullet experiencing increased yaw (up to 11 degrees). This is something that I�ve never considered; but the paper does say �Based on the totality of the evidence failing to support the Litz hypothesis, it seems that the Litz hypothesis of yaw-induced drag increase is an unlikely explanation of lower BC in most cases.�

I was most amazed at the variation (up to 20%) in BC between different lot numbers and rifles. I guess we can all go back to selecting a bullet based on its intrinsic appeal, not a �scientific� BC number.

And..... actually get out there and shoot them to get an idea of where they will hit. For most of us, we are just trying to be able to routinely hit something the size of a basketball at ranges inside of 400yds. I like big BC #'s, retained speed, and less wind drift, but what happens after the bullet lands on the target is still more important to me than gaining a "2.9 inch advantage in drift at 387 yards in a 10mph wind".

I draw just the opposite conclusion. The variation in BC between different lot numbers and rifles demonstrates just how sensitive the measurement is. Or put another way, just how easy it is to screw up the aerodynamic properties of something traveling at supersonic speeds.

As others have stated, the manufacturer's BC numbers are a good starting place, but if you are into longer range shots, then test firing and adjusting the BC to match your observed drop from your load and from your gun will get you closer to the real value, assuming you do your testing carefully and accurately. Why bother? Well with an accurate BC you can now predict how your load behaves in conditions other than what you tested at. I don't know about you, but my test range is often at a lower attitude then where I hunt and it always seems to be either way hotter or colder hunting than when I was at the range. Knowing the BC lets me figure out how those changes affect my load.

Great post JPro!

Only thing that I tend to look at these days on BC is comparing bullets.

I also know enough to be wary of something that sounds to good to be true....

But we have to compare somewhere for what we want for longer shots... BC is a good place to start.

Or at least know about what bullet weight and that I want a BT design not a round nose...

Okay, you're saying the drag coefficient of different shapes varies alike through that range of speeds.

As for SD affecting drag, that's not so, but I don't think that is what you meant to say.

SD is constant for a given bullet regardless of any speed we're talking about, so the drag coefficient is the only thing in play. If bullets with shapes as different as the G1 and G7 standard projectiles retain a nearly constant drag coefficient ratio from Mach 1.7 to Mach 2.6, then the same it's likely true for other shapes. However, I've only done the work for G1 and G7. That's an important comparison because there are those claiming the G7 BC produces more accurate predictions of trajectory, but I've shown that you can't tell the G1 and G7 apart between Mach 1.7 to Mach 2.6, which is a prominent part of the velocity range found in most hunting situations.

My first thought is that I hope their numbers aren't representative, since I have a couple of bags of .308 150 gr E-tip blems among my reloading supplies. lol

Looking around a bit, I ran across this thread:


I guess that result is in line with the study, but it'd be nice to see the test repeated with more testbed rifles. That said, I wouldn't be surprised if the .469 BC was inflated. I read elsewhere that it was based on a computer model, not testing. It is pretty funny that there was only one underestimate of BC on that chart... Even the Barnes 168 TTSX came in over 20% below the published BC (again with a sample of one rifle/barrel).

I'd like to shoot some of the 150s at long range and see how the BC compares to the advertised number. A 400 yard milk jug experiment using a 308 Win would be good as well. I think I'll stay away from them in the 308, and go with either the 30-06 or 300 WSM when using this bullet.

A little easier method of estimating the BC would be to chrony your load, shoot it at various ranges measuring the drops, then back the BC out using ballistic software. You'd need to know the temperature, altitude, humidity and wind conditions. Given the rifle-to-rifle variation, what really matters is how the bullet shoots in your rifle.

A big time amen to that!

Dober

Kman-my curious side has me wanting to ask so I will but how far off are you planning to use this bullet?

Thx
Dober

THNX - That's very, very interesting.

IMO (only) using i.e. round nose vs spitzer there will be a diff in trajectory at the SAME vel. due to wind RESISTANCE and drag?? huh?

I'm glad to see this thread. Working with my 6.5-284 a couple of years ago, shooting 140 gr. SST's at 2,950 fps (chrono'ed), setting up a turret and recording the clicks for actual zero on targets out to 600 yds, I noticed that the ballistic program projections weren't right. When I extrapolated the observed data back into the formula, I came up with a B.C. much lower than advertised.

DF

In practical terms a round nose bullet will have a lower BC than a spitzer bullet in the same caliber (not just bullet diameter, but the same caliber as in .308 Win). Thus, the round nose with its lower BC will have a different trajectory and wind drift. However there are exceptions and bullets with the same BC have the same trajectory and wind drift regardless of their shape. This is possible because the bullet with the less streamlined shape makes up for it with greater sectional density (weight), which is the other part that makes up the ballistic coefficient.

I'm shooting a Federal factory load (P7RH) in a 7mm Rem Mag. The factory specs are 3025 fps, 150gr Nosler Ballistic Tip with a BC of 0.495. My altitude was 2530 feet with a temp of 65 F. I zeroed for 200 yards and found I had a drop of 19.5 inches at 400 yards. According to the software and the factory specs I should have a drop of 18.0 inches at 400 yards.

Extrapolated the observed data back into the formula and coming up with a corrected BC is just too much like work for me. I just put the values in Ballistic Explorer and use the slider controls to adjust BC and/or some combination of BC, MV, Temp, head and tail wind to get the observed drop. That way It's more like a video game than work. The software holds the observed zero at 200 yards while I click the slider controls until it reads 19.5 at 400 yards. For trace 1 I changed just the BC and it came out to 0.387. For trace 2 I dropped the velocity to 3000 fps, dropped the temp to 63 F, slid in 5 MPH of head wind and then clicked the BC down until I got 19.5 at 400 yards, which turned out to be 0.411. Trace 3 shows the factory values.

I have 40 loaded in my 300 wsm and I'm sure they will be fine out to 300 or so but don't think I will advise them to my friend at 308 speeds.

That was a good demonstration of just how easy it can be to fine tune a load's BC based or real world conditions and drop data. Even with such demonstrations, I expect the technique will remain one of the best kept secrets among long range hunters.

That was a nice demonstration, a lot more sophisticated than my plugging numbers until the ballistic program reflected my data. Just goes to show, one cannot always rely on marketing hype. That's another reason why one needs to go to the range and shoot each distance involved when having a yardage turret made for a certain load. They'll go by the advertised B.C. and the finished product may not be right, probably won't be.

DF

No need to test every range unless you think your turret is crap, and if you do, get a better one. Just zero at some range like 200 yards and then see what drop you get at two more ranges such as 300 and 400 yards or longer if that's what you intend to shoot at. From my prior post you see it's easy to find the BC that gives you the right drop at one other range. The magic of Dr. Oehler's software is that you can use drop at a third range to test that BC or discover other factors like small changes in MV that give a match. Every time you click or drag a slider control all the numbers and graphs update instantly, you just click-in the solution. No other software that I know of figures head and tail winds into BC, yet I run into both most of the time when I'm shooting. Once you have the real BC worked out you can figure your drop at any other range under any other conditions. Unless you're 12 and rich you don't have enough time or ammo to test all that at the range.

Unfortunately, MacLorry is right, this technique and many others might as well be a secret because the required software is beyond the means for most of those reading this blog.

The scope and turret weren't the weak link in my chain, it was me...

Shooting at hundred yard increments gave me the confidence I needed in the rig.

If I was sophisticated enough with programs, etc. I'd have done something like you recommend.

DF

[/quote]
From my prior post you see it's easy to find the BC that gives you the right drop at one other range. The magic of Dr. Oehler's software is that you can use drop at a third range to test that BC or discover other factors like small changes in MV that give a match.
[/quote]

Thanks for the credit, but the "sliders" and easy solution were entirely the invention Richard Larson who has labored on "Ballistic Explorer" for many decades.

Ken O

Thanks, Dr. Ken, for your input, as always.

Wishing you and yours a wonderful Christmas season.

Best regards,

DF

Appreciate your reply and enjoy learning something about who�s behind the scenes. I just read Guns & Ammo�s interview with you in their January issue. I was one of those fortunate to experience shooting with a gun club buddy�s Model 43 some 17 years ago. Back then we thought 300 yards was long range and I see from the G&A write up you�re planning something for much longer ranges, a spin-off of a military system. Don�t know if you can share anything more, but if this topic is any indication such a product is highly needed.