An eye opener huh?? Who can you believe?

POP,

Thanks for finding that page again. Is it the entire page however?

I am pretty sure that is the whole thing.

Discussing just three bullets the Sierra .308 dia 180 gr SBT has a listed BC of .501 and Jamison measured .426 or 85%.

The Sierra 165 gr SBT is listed at .404 and Jamison says it's .383 or 94%.

The Sierra 150 gr SBT is listed at .380 and Jamison says it's .361 or 95%.

Using common sense the 180 SBT does not seem to be that much different in ogive or meplat thus the three bullets should be closer in BC.

I would really like some accurate numbers.

For general-purpose big game shooting it's not too crucial under 200 yards. Beyond 300 yards it begins to come into play with increasingly greater force with each yard. It's simply the aerodynamics of the bullet's shape. A bullet with a high BC retains it's velocity better, drops less at given yardages and is less effected by the wind.

POP,

I did not forget the length. My statement "using common sense" covers the length and the weight.

I tend to believe Jamisons numbers at least in proportion. This throws doubt on Sierra's numbers.

Pop, length has almost nothing to do with BC.

Good point, Savage99. Sierra's numbers are basically saying the 180 has a better form factor than the other two. It does have a longer boattail. Rick's test showed that it did have a better form factor, just not as much better as Sierra advertises--at least not in Rick's rifle.

I hadn't noticed before (never used that bullet) that Sierra's advertised number is so close to the Nosler BT in that weight. The other two are advertised well below the BT. It's definately something I'd keep in mind if I was going to use that bullet.

On the other hand, it tests the same as the XBT advertised at .550. So if Sierra is exaggerating a little...well, everything's relative.

I am not sure where to turn next. I looked at some online BC calculators and have Ingalls charts. But just one slight choice up or down while comparing an ogive and your 10% off anyway.

It's a bitch ain't it?

Savage, in regards: "Discussing just three bullets the Sierra .308 dia 180 gr SBT has a listed BC of .501 and Jamison measured .426 or 85%.

The Sierra 165 gr SBT is listed at .404 and Jamison says it's .383 or 94%.

The Sierra 150 gr SBT is listed at .380 and Jamison says it's .361 or 95%."

If you accept the idea that the bullets above have the same form(which is not precise) then you are left with the weight issue. For the same caliber and form, BC is directly proportional to SD, or weight. As previously mentioned, LENGTH has zip to do with BC. It is all about form and weight. Further, the numbers in Pop's excerpt are all based on the G1 form and that skews the data a bit. I suppose that since they all get the same "skewing" you have a relatively skewed up pile of data there.

You will get a better idea of BC by comparing bullets of the same weight, not same form w/ different weights. From my recollection, bullet nose shape is responsible for about 75% of drag, the shank less than 5% and the remaining 20% to base design. If you really want to know what a bullet will do, you need to test it yourself. Your gun/barrel/weather/elevation, etc, etc, will affect the numbers advertised by the bullet makers, and the gun writers. Sometime back there was an article about the Swift 7mm Scirocco(150 gr) that had to do with observed BC over an Oehler PBL. Using the G1 form as point of reference the writer/PBL was generating BC's in excess of 1.0. That would be cow flop and we all know that. Point is, you got to seperate the beef from the crap sometimes.

That is all.

Dan:

I see what you mean. I sould have stated weight...not length. The way I tought about it is if you have 2 similar shaped bullets (form factor) then due to the extra weight ithe heavier is longer. Should have said weight!

Well Pop, if they're the same caliber that is correct, and we've all done that somewhere or another. The longer I look at ballistics though, the more I recognize that much confusion arises when words are used with improper context, or in improper places. It creates legends sometimes. Like: "What caliber is your gun chambered in?" Are you saying accuracy but really mean precision? When you say 'Overall length', do you mean case, bullet or a loaded round? Diameter means Groove or Bore? It's a mine field out there!



I saw a thread about a year back here at AR, two fellows having a damn fine cat fight about twist rates and bullet RPM. Funny thing is they were both arguing the same side just using different terminology, and not all of it right.



I will say sincerely though, I've never phucked up once here at AR.



I guess I'll say these last few things on the subject of BC's. I don't have all of it figured out yet either, but I'm still working on it. As was stated several times above, inside of 300 yards and for a given bore, the differrence in RN and Spitzer BC's isn't truly significant. Form or weight has to be radically altered to generate substantial differences. Beyond 300 yards or so, and that is just a guideline not a benchmark, small things mean a lot. IF all other things are equal, I will select the higher BC bullet when I have that choice to make, not because of trajectory, but because of wind drift. Unfortunately bullets and circumstance are not equal under the law of man or nature. The quality of BC does not necessarily mean a sleek bullet. Take a look at this web site that Brent linked on one of the other forums, maybe Cast Bullets, not sure. Look at the BC that is projected for it... http://www.iastate.edu/~codi/PPB/gd-ppb.htm



Good shooting to all.