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

"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.