Actually, BC is a direct function of the weight of the bullet. Length has little to do with it (except that heavy bullets tend to be longer, aerodynamic shapes--long ogives, sharp points and boattails--tend to make the bullets longer). The length of the bullet itself, with other factors held constant has only a small contribution to drag.
The BC of a bullet is directly proportional to its sectional density which, in turn, is directly proportional to its weight.
What this means is that if you have two bullets with identical shapes but one is twice as heavy as the other, the heavy one will have a BC exactly twice as high as the light one.
The BC isn't just a measure of how much aerodynamic drag a bullet has. It's a measure of that as well as how well the bullet will resist this drag.
So think about the bullets above, with identical shape but one is twice as heavy as the other. They will have the same amount of drag. The air will slow them down by applying the same force of drag. It will change their velocity by decelerating them (or causing negative acceleration). This acceleration is equal to the force divided by the mass (per f=ma).
So with the same force of drag applied, the heavy bullet will be decelerated at only 1/2 the rate of the light bullet. It will hold onto its velocity twice as well as the light bullet...thus, the twice as big a BC.
Clear as mud?
That's why I've been toying with the thought of getting into swaging one of these days...and messing with Tungsten core bullets. Tungsten is 1.7 times as dense as lead, it's pretty cheap and widely available. Can anybody say, "30 caliber, 300 grain bullet with a BC of over 1.0?" [img]images/icons/grin.gif[/img]
Anyway, here's what this means in practice:
When comparing bullets with similar shapes, the lighter ones will have lower BC's.
Performance-wise, the lighter bullets at higher velocities will shoot flatter until the heavier bullet "catches up." But often the heavier bullet won't "catch up" trajector-wise until a range far beyond your maximum.
So why use the heavy bullet?
First reason is wind drift. Given similar shapes, a heavy bullet will always drift less in the wind than the lighter bullet even with its lower velocity. BC is everything when it comes to wind drift. You won't "make it up" with extra velocity, even at relatively close ranges. With a rangefinder and a drop chart, drop can be accounted for. The wind on the other hand is much more difficult. Having a bullet that only drifts 1/2 as much will make whatever skills you have at reading the wind that much better.
The second is retained energy. The people here aren't just punching a hole in a piece of paper. If your "super-duper-ultra-wizbang" magnum only hits as hard as a 22LR way out there, it won't kill very well. The heavy bullet with the high BC will retain more energy and hit harder way out there.
An exception to the rule is when you compare bullets with different shapes. A light bullet with a much more aerodynamic shape, such as Warrens, or GS's (possibly, I don't know how accurate his advertised BC's are) launched at a high velocity can outperform a less aerodynamic heavy bullet in all areas out to your maximum range. In that case, you have the benefit of a much flatter trajectory which could help if you find yourself hunting "medium ranges" without a rangefinder.
Warren, anybody, feel free to correct anything wrong I've said above. I'm not an accomplished long range shooter like many on this site...just a ballistics nerd!
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>
Actually, BC is a direct function of the weight of the bullet. Length has little to do with it (except that heavy bullets tend to be longer, aerodynamic shapes--long ogives, sharp points and boattails--tend to make the bullets longer). The length of the bullet itself, with other factors held constant has only a small contribution to drag. <HR></BLOCKQUOTE>
Not saying that I'm not wrong in one sense or another, but then exactly how do you explain the bullets like the Lapua Scenar 155gr? AFAIK (I haven't personally vivisected one) they have a BC very close to a 175gr SMK, for instance, and a similar lenght. But one is 20gr lighter than the other. The 155 Scenar is as long as it is because the nose is effectively hollow, due to the lack of lead to make weight. This would seem to indicate to me that the length of the bullet, and more precisely how its shape corresponds to the 'ideal' bullet (G1 or whatever it is) that affects the BC. The weight would seem to be a side effect of filling up the voids w/ lead.
Ok I begining to get the picture. <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> <HR></BLOCKQUOTE>Clear as mud. Hey mud can be fun to get into. Now this swagging of your own bullets will materials of the same weight produce the same lenght bullets [img]images/icons/confused.gif[/img].
just want to make sure I got it right. Learning is fun even by long distance
Courage,Spirit and Honor
Pass It On
Would like to step in and offer a compromise. Based on what we know today and how we make stuff, the above is "correct". However, change the parameters (rules) and you change the result.
The one thing that doesn't change...much, are the laws of physics. Aerodynamics doesn't care how you part the air. It does react basically the same.
So, refer to ballistic programs and add in different parameters like BC, vel, and range. You will see the affect of all of these on the energy and vel. at the impact range. They are interconnected and all are a compromise. You will find that high BC will have a tremendous effect on retained energy, impact vel, and wind drift. You can't launch a blunt object fast enough to be of any use at long range.
With high BC bullets from 7mm to 338 and with the muzzle vel. we are using, any critter we hit is going to die and quickly. The compromises are the max range and size of game you intend to shoot at. Also recoil, rifle weight, cost and personal preference, are all deciding factors.
This thread started by asking what to choose in a calibre. My answer is choose the largest case and longest barrel to launch the highest BC bullet at 2900fps and above accurately. When you factor in rifle weight, cost and recoil, your choices will quickly reduce to the few that are mentioned here lots. When they figure how to build bullets out of another lighter material, the whole mess change again.
Interesting topic, sure to get a bunch of different opinions & answers.
My quest for a new LR rifle & caliber began with a comparison. I wanted a rifle primarily for conventional LR matches (fired from a sling, not a benchrest) that would out-perform a 6.5-284. It had to be able to:
1. Fire a very high BC bullet
2. At high velocity (3,200+)
3. Have moderate recoil WITHOUT a muzzle brake. (they're not legal in conventional LR)
4. Have the potential for excellent accuracy.
Due to the recoil, a 30 or 338 variant was likely out of the question due to the weight & resulting recoil of 220-240 & 300 grain bullets at high velocity without a brake. The Lazzeronis intrigued me when they came out, but one call to their shop years ago deterred me from using their stuff.
I saw Berger's 7mm 180 VLD & was intrigued. It had an advertised BC of .698 & could be fired at high velocity in a 7mm STW. I was leery of reloading a belted magnum, so I held off. Once Remington introduced the non-belted 300 Remington Ultramag, I hoped they would consider making it in 7mm, & as we all know - they did.
This allowed a lighter, but very high BC bullet to be fired from a non-belted magnum case which had some ballistics surpassing a 6.5-284 according to my ballistcs program. Velocities are in the neighborhood of 3,300- 3,400 for a 180 at the muzzle, 2,500 at 1,000, & 1,500+ at one mile. Superb wind resistance, fast, & FLAT. I fired a buddy's Savage lightweight in 300RUM from a sling with 175 SMKs & was surprised at the recoil - not bad at all.
After talking with several people including Walt Berger about it caused me concern over whether his 180 would hold up at those velocities (and as he was patient enough to explain: RPMs) He said it was very iffy if the J4 jacket would stay together & wasn't designed for that application. He suggested some other VLD makers using thicker jackets to try. Jimmy Knox & Bob Cauterucio were equally as patient & helpful, I've tried bullets from both & have been extremely impressed with their quality & uniformity.
Powders were another story. Almost no load data existed for a 7mmRUM. Few companies had quality dies either. I bought a few powders while the rifle was being built, gathered what data there was, talked to some knowledgeable folks, & started putting the pieces together. After frustrations with 'traditional' powders I've been trying some 50BMG powders & 3 bullets: Cauterucio 156 & 176, plus JLK 180s.
Since it will be fired from sling supported position, it's built like a very large international smallbore rifle: weight & ergonomics to absorb recoil (adjustable buttplate, cheekpiece, handstop, etc.)
So far, it's been one trial after another, but the pieces are starting to come together. Groups (when I do my part) are getting consistently sub-MOA at 500 with one showing potential to shoot @ 2" at that distance. It's comfortable to shoot even without the brake, & should do well for LR coyotes, & if I can swing it eventually, bigger game. LR benchrest would be fun to try, even though it's not really designed for it.
Time & shooting will tell. I'm hoping to make a 1,000 yard match next weekend. Although I've shot in some in the past, it was with 308s & 7mm-08s. I may have to have a chiropractor on retainer, but it ougta be a blast!!! [img]images/icons/grin.gif[/img]
As far as the BC question, I'm no expert (or even gifted amateur) but I learned a little talking with those bullet makers. These three, & a few others (Warren Jensen included) have forgotten more about bullet construction, performance, & dynamics than most of us will ever know! Sectional density, weight, & construction play into it, but design of the ogive characteristics & even the BOAT TAIL play into it. That's why bullets of equal weight can have significantly different BCs. Design. The cutting edge stuff from LRBT & PRL made from solid, super dense, one piece construction are potentially a new era....
Best of luck, I hope your projects aren't as much a pain in the @$$ as this one's been. [img]images/icons/smile.gif[/img]
I'm not seeing a 155 Scenar on Lapua's site. What is the advertised BC? While comparing the other weights of Scenars with SMK's, they seem to be fairly similar weight for weight. If they make a 155 with a similar BC to the 175 SMK it comes from the shape (probably a really long--13 caliber or more--secant ogive with a very sharp tip). If they filled the nose full of lead without changing the shape it would have an even better BC.
Using the same materials the bullets would have all the same weight/length limitations. But you could make things that aren't on the market. A long 220-240 grain bonded, partitioned bullet with a high BC wouln't sell enough for a manufacturer to produce. But wouldn't it be cool? Using Tungsten can make much heavier bullets with the same length. Don't get me wrong, I'm not ready to try this tomorrow, I've just thought it would be a fun thing to do "someday." To be able to make a bullet that isn't on the market...to feel the same satisfaction knowing you made the bullets as you feel shooting reloaded ammo instead of factory ammo. It just seems like a fun hobby if I ever get the spare time.
Sorry about being a numbers guy...that's just the way I am. [img]images/icons/grin.gif[/img] Time of flight is important, but more for trajectory than wind drift...let me try to just give you a real world example to show what I'm talking about.
Say you have a 300 RUM with a long barrel. You can shoot the 180 SMK's at 3450, and the 240's at 2950.
Trajectory-wise, the 180's beat the 240's all the way out to 1900 yds before the 240's "catch up." So if you only hunt at less than 1900, the 180 is flatter shooting. The added velocity "makes up" for the lower BC as far as trajectory is concerned. This would be very adventagous to a "regular" hunter that doesn't use a rangefinder and just wants his rifle to be really flat shooting. In fact, even a lighter bullet will do much better at closer ranges for such a hunter.
But you have a rangefinder. You will know the range within +/-1 yd. You have verified your drop chart and it will be accurate to your maximum range. The "flat shooting" advantage kind of goes away. A couple of extra clicks on your scope are the only difference.
What's the biggest worry after that? Wind drift. The 240's drift less from the beginning. At only 300 yds they drift almost an inch less in a 10 MPH crosswind. At 500, the advantage is 2.5". At 800 the advantage is more than 7". At 1000 the advantage is over a foot. And on and on. If you can correct for trajectory, you'll be more accurate with the 240 unless you are shooting in a vaccuum. The added velocity of the 180's didn't "make up" for the lower BC here. The 240's will always drift less.
Then there's the energy. In this example, the 180's actually start out with 120 ft-lbs more. But that advantage is gone before 100 yds. At 500, the 240's have the advantage, 3100 to 2680. At 800 it's 2400 to 1870. At 1000 yds it is 2000 to 1440. That's enough difference to make a difference in killing power.
The 180's shoot flatter. But you can adjust for that very accurately. The 240's resist the wind better (much harder to adjust for) and hit much harder at all ranges.
Anyway, I hope that gets the point I was trying to make accross a bit better than theoretical ramblings....