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Old June 16, 2020, 03:50 PM   #1
gbclarkson
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Question(s) about bullet measurements

Bored at home, I was cruising through CTD's .357 mag for sale. Not going to buy any - really expensive right now; but I came across two measurements that have me wondering: sectional density and bullet ballistic coefficient. For reference, S&B .357 advertises a sectional density of 0.177 somethings (no units included) and Magtech sports a bullet ballistic coefficient of 0.149. What are these? I suppose I could Google but I really want to know is: Why should I care? Is 0.149 better than 0.170 (which is the ballistic coefficient of Winchester Super X, 158 grain) if the two bullets are the same price?

Pointless questions, I know. But I'm really bored.

Does anyone use these figures?
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Old June 16, 2020, 04:56 PM   #2
Jim Watson
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Not on pistols.
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Old June 16, 2020, 08:36 PM   #3
Mycin
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Sectional density is calculated as the bullet weight (in pounds) divided by the bullet diameter (in inches) squared. So for a 158 grain .357 bullet, it's (158/7000)/(.357*.357) = 0.177. Heavier bullets for a given caliber yield a higher sectional density, and all bullets of the same weight in a given caliber will have the same sectional density. It is useful as a rough indication of penetration potential. I also find it useful when investigating a caliber that I'm not that familiar with, to quickly compare bullet weights in that caliber to another caliber that I already know about. For example, in 480 Ruger (.475in diameter), a 280gr bullet would have the same sectional density as a 158gr .357 bullet. In 41 Magnum, it'd be a 208gr bullet.

Ballistic coefficient is an indication of how efficiently the bullet cuts through air/wind resistance (higher is "better"). It's more of a measured value than a calculated one, as it depends on bullet shape, length, diameter, composition, and probably other parameters that I'm not aware of. Indeed, it seems to be different at different velocities for the same bullet. BC is not nearly as useful for pistol bullets as for rifles, especially long-range rifle, but I've used it in online calculators that purport to predict bullet drop at various ranges to get a feel for how much a given load is likely to drop at range from my .357 carbine. At any rate, it's most useful for long-range shooting, whether that's a 150-yard pistol shot or a 1,000 yard rifle shot. For typical pistol ranges, it doesn't mean much.

Hope this helps.
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Old June 16, 2020, 08:49 PM   #4
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I'll let you look up the formulas but essentially one uses bullet size (width and length) and weight to give you a number indicating its relative tendency to keep going, and the other uses the bullet shape and weight to give a relative number about how well it slips through the air.

These numbers have a practical use in rifle shooting, not so much in pistol, outside of specialty long range pieces.

Regular pistol bullets usually have BC's between .1 and .2, while rifle bullets can be higher, .4 or .5 something and the best long range bullets are around .6 something, (not counting the VLD type introduced in the last couple decades)

The numbers don't really mean anything by themselves, what they do is give you a relative comparison scale for which bullets will tend to penetrate more, or lose velocity sooner and drop more than others.

That's the quick and dirty, and leaves out a bunch of stuff, and all the math.
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Old June 17, 2020, 08:55 PM   #5
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SD only considers weight in relation to diameter. Everything else being equal higher SD's mean better penetration. All 180 gr 30 caliber bullets have the same SD regardless of bullet shape.

BC is a mathematical term describing how aerodynamic a bullet is. Long bullets in relation to diameter also tend to be more aerodynamic, but shape is also a factor. There can be a HUGE difference in the BC between a 180 gr round nose bullet and the very high BC 180 gr bullets.

With rifle bullets it matters. For example a 30-06 shooting one of the better 180 gr bullets will be moving at the same speed at only 75 yards as a 180 gr round nose bullet fired from a 300 WM. And the farther you go beyond 75 yards the bigger advantage 30-06 has. A 308 shooting a high BC bullet will pass 300 WM at 175 yards.

BC is rarely a factor with handguns. But SD can be.

As said earlier longer bullets in relation to diameter penetrate better. In the same caliber heavier is always longer and will always penetrate deeper. All other things such as bullet construction and impact speed being the same. But when you compare bullets of different calibers things get interesting.

For example a 147 gr 9mm bullet has a SD of .167. A 180 gr 40 caliber bullet has a SD of .161, and a 230 gr 45 bullet is .162. They are virtually the same with a slight advantage to 9mm. In real world situations penetration and performance will be about the same.

The 158 gr 357 load you looked at with a SD of .177 should penetrate a bit better than all of the above. But the difference between .177 and .167 isn't huge. Rifle bullets can range from .150 to well over .300. That makes a huge difference when shooting larger game. Most hunters want a SD of about .270 or greater for big game like elk, moose, etc.
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