Home Forums Hunting & Shooting Ask The Gunwriters Inflated BC? Specifically the 150gr etip?

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.