Re: Heavy for Caliber: Definition
Good questions, actually.
Let's take 2 calibers that are well represented here; .223 Remington (5.56 NATO) and .308 Winchester (7.62 NATO).
The original .223 used a .75 inch 55 grain bullet with originaly a 1:14 twist then a 1:12 twist. Some years later, the bullet weight grew to 62 grains but since it had a steel penetrator, the bullet was longer because of the lower density of the steel. These bullets are fine for the original intent which was short range, 300 yards and under.
The 7.62X51 (T-65) .308 Winchester was originally loaded with a 147gr bullet (I forget the length) and it did well for its intended use, up to mid-range (600 yards and under).
When people started wanting to shoot longer distances, the low ballistic coefficient of these original bullets was working against them. In order to reah longer distances. you can try to push a bullet as fast as you can, but if the bullet has a low BC, it simply sheds this extra velocity faster. So, you try to increase the ballistic coefficient of the bullet such that it will retain more of its initial velocity longer.
The ballistic coeeficient is a function of sectional density and shape of the bullet. Sectional density is a function of the weight of the bullet in a specific caliber. In order to increase the sectional density of a bullet, you can make it heavier (and if it's in the same caliber, it will be longer) or you can use materials than have a higher specific gravity (density) than jacketed lead. Depleted uranium works well for that, as does gold, platinum, carbide, tungsten, etc. Consult you nearest periodic table and alloy manufacturer. The point is that we are currently stuck with jacketed lead, unless you have access to materials that mere mortals do not. So we make the bullets longer.
Back to our .223. If you use an 80 grain bullet, the SD for that bullet will jump from .157 for the 55gr bullet to .228 for that 80gr bullet. The BC value will go from .243 to as high as .510 for the 80gr bullets I use. This is such a big increase that even though my 80gr start out almost 400FPS slower than the 55gr bullets, by the time they get to 500 yards, my 80gr bullet has overtaken the 55gr bullet, which is now falling on the ground as it goes subsonic, my bullet will stay supersonic past 1000 yards.
On the .308 front, the 155gr bullet is making use of its very aerodynamic shape to increase its BC value. Remember what I said earlier, the BC is a function of the SD and the shape. The 147 had an SD if .221, which is pretty good but its shape was not very aerodynamic. The 155SMK has an SD of .233 and a listed BC between .430 to as high as .504 above 2700FPS. This bullet is very streamlined. The 175SMK has a BC about the same at the top end as the 155 Palma, but as the velocity reduces, it has a higher BC value, because its sectional density is higher that the 155gr Palma. This tells you that streamlining or increased SD can take you further by increasing the BC, but a combination of streamlining and especially an increased SD will do a better job over the longer distance. In other words, substance is somewhat more important than style, but you still need style.
And for the long distance, you still need a certain muzzle velocity to have the BC work for you. Finally, launching a 200gr bullet with a high BC, will require a minimum MV so that the recoil might be more than you want to deal with.
Trade-offs, everywhere.