History also plays an important role in the perception of ‘proper’ bullet weights for .30 caliber. As pointed out by Dr. K. C. Erikson in 1995 [Ref5], and more recently by German Salazar, .30 caliber shooters used ~173 grain bullets as their standard for many decades before long range shooting became popular and the modern push towards really heavy bullets came about.
Ballistic coefficient is comprised of three components: mass, cross sectional area, and drag [Ref1]. Mass was discussed in the previous section. The cross sectional area is related to the caliber of the bullet. The drag of the bullet, measured by the form factor, is a big part of the problem with .30 caliber bullets.
The aerodynamic drag which acts to slow a bullet down is related to how streamlined its profile is. Just like a Corvette has less wind resistance than a VW Bug, so a long sleek VLD with a boattail has less aerodynamic drag than a short, fat, flat based bullet. If two bullets have the same mass and diameter (same sectional density), the one with less drag will have the higher BC. The ‘drag’ part of the BC equation is quantified by the form factor. The form factor simply relates the drag of the bullet to the drag of some standard bullet. For this discussion, I’ll refer to the G7 standard because it’s more appropriate for long range bullets than the classic G1 standard [Ref4]. A bullet with a G7 form factor of 1.000 has exactly the same drag as the G7 standard projectile. A G7 form factor less than 1.000 means the bullet has less drag than the G7 standard, and a form factor greater than 1.0 means the bullet has more drag than the G7 standard. A bullet’s BC is simply its sectional density divided by its form factor.
Let’s take a look at some of the “heavyweight” bullets in various calibers and see what their form factors are. Figure 2 shows the profiles of some popular heavyweights in 6mm, 6.5mm and 7mm along with their G7 form factors, sectional density, and G7 BC. The BC data presented in Figure 2 was measured using a technique of proven accuracy and repeatability. The details of the BC testing are not important, it’s just important to note that these numbers were all measured in the same way, and were not obtained from the manufacturers [Ref4].
So what do the numbers tell us? Well, the G7 form factors of the 6mm bullets are right around 1.0, which is pretty good. The form factors of the heavy 6.5mm and 7mm bullets are on average less than 0.95, which indicates extremely low drag. The G7 form factors of the more blunt .30 cal heavyweights are ~1.08 average. That’s about 3% higher drag than the 6.5mm and 7mm bullets. 13% more drag is a huge deal. It would mean 13% lower BC if the bullets had the same sectional density, but the heavy .30 caliber bullets have about 9% to 13% higher sectional density than the 7mm and 6.5mm bullets, respectively. The result is that the heavy, blunt .30 cal bullets have a BC that’s only marginally greater than the 6.5mm bullets, and about equal to the heavy 7mm bullets.
One more thing to consider about aerodynamics is the effect of aftermarket bullet modifications, specifically, pointing. Bullet pointing reduces drag more for smaller caliber bullets than for larger caliber bullets. The reason is because nominal meplat diameters are proportionally larger on smaller bullets, so reducing them helps more. For example; a 0.065" diameter meplat is only 21% of .308 caliber, but it’s 27% of the 6mm caliber. Squeezing the meplat down to 0.040" makes it 13% of .308 caliber and 16% for 6mm caliber. The difference doesn’t seem like much, but there are two things to remember. First of all, the area of the tip is what’s important, and the area scales with the square of the diameter (meaning the smaller caliber has even more of an advantage than indicated by the above numbers). Second of all, the smaller bullets tend to operate at higher average speeds than the larger bullets. The reduction in wave (supersonic shock) drag is more significant for the smaller bullets traveling faster. Effects of bullet pointing are brought up because it’s another variable in favor of smaller calibers. However, the rest of the discussion will go back to considering unmodified bullets.
A Closer Look at Recoil
I’ve mentioned recoil as a negative effect of the larger calibers, but the subject warrants a little more discussion. The following discussion is about recoil in general, and is not specific to the .30 caliber.
There are basically two different ways in which recoil is bad for accuracy and precision. The first is the effect it has on the mental state of the shooter, and their ability to deliver well executed shots. Many shooters develop a “flinch” from anticipating the heavy recoil. This is a problem that affects shooters to various degrees, depending on mental discipline, physical size, etc. Heavy recoil also has a way of “loosening up” a position, requiring the shooter to re-adjust periodically thru a string of fire. This seemingly minor inconvenience can prevent a shooter from shooting as fast as they would like. Speed can very often be of the essence, especially in benchrest shooting where you don’t have to wait for pit service.
The second aspect of heavy recoil is the effect it has on the rifle itself. The high pressure and heavy masses moving around tend to set the rifle in motion early (before the bullet exits the muzzle) more so than a smaller caliber shooting lighter faster bullets. German Salazar describes this as “barrel movement during barrel time.” It reasons that when shooting such heavy recoiling rifles with slow heavy bullets, that accuracy is much more sensitive to the quality of the shooter’s hold, trigger squeeze, and most importantly Natural Point of Aim (NPA).
To sum up: heavy recoiling rifles are harder to shoot accurately. Even if a shooter overcomes the mental aspect of heavy recoil, the “system” is more sensitive to minor imperfections in shot execution. This may be another reason that drives .30 cal shooters down to the “middleweight” 190 grain class bullets instead of the proportionally heavy 220-240 grain bullets.
Note: the above discussion on recoil is most pertinent to prone target shooting where the rifle has to be supported by the shooter. Benchrest shooters who use heavier rifles supported by steady rests are not as subject to the “barrel movement during barrel time” gremlin as prone shooters, and that may be why .30 caliber hasn’t fallen out of favor for benchrest as much as prone shooting in recent years.