Bullet Weight and Scaling
The May 2007 Issue of Precision Shooting [Ref1] featured part one of a series authored by yours truly that focused on the effects of scaling bullets. The mass, ballistic coefficient (BC), stability, velocity, recoil and other effects were described. For this discussion, I would like to focus on bullet mass, and how it’s affected by scaling between calibers.
It’s a generally accepted fact that the heaviest bullet in a given caliber is the best bullet to use for long range target shooting. There are several credible studies of this topic, [Ref2] [Ref3] and it is one of the fundamental truths of long range ballistic performance. Assuming constant form factors (drag profiles), heavy bullets will have higher BC’s than lighter bullets of the same caliber. Heavier bullets will also have lower muzzle velocities than lighter bullets, but when loaded to the same pressure, the higher BC of the heavier bullet is more valuable than the higher muzzle velocity in terms of retained velocity and wind deflection at long range. German Salazar put it aptly: “Muzzle velocity is a depreciating asset, not unlike a new car, but BC, like diamonds, is forever.†For this reason, the present discussion focuses on the heaviest bullets available in each caliber.
Figure 1 shows how bullet weight is affected when you scale the well known “heavyweight†6.5mm 142 grain Sierra Match-King bullet up and down in diameter. You can see that a “heavyweight†.224" bullet is ~89 grains. Makes sense, as 90 grains is the heaviest .224" bullet available. The heavy 6mm bullet is 112 grains. Ok, we know that 115 grains is about the practical upper limit for that caliber, and there are several fine offerings from Berger and DTAC in this weight range. The 142 grain 6.5mm that was chosen as the basis of comparison is in good company. In that weight class, you have the Berger 140VLD, 140BT and 140 Short BT, the Sierra 142 MK, the Hornady 140 Amax, the Lapua 139 Scenar, and the 140 grain JLK. That’s 7 legitimate “heavyweight†bullets in 6.5mm. Move up to 7mm where the “heavyweight†is supposed to be 177 grains, and you have the Berger 180 VLD, the Sierra 175 MK, and the JLK 180 VLD. Now move up to .308 caliber. According to the established trend, a real “heavyweight†.30 caliber bullet should weigh 229 grains. How many .30 cal bullets are that heavy? You’ve got the Sierra 220 and 240 grain MatchKings. The next heaviest things are the Sierra 210, Berger 210, Hornady 208, etc. So there are only two bullets, the Sierra 220 and 240 grain MatchKings that are even in the neighborhood of what a “heavyweight†.30 caliber bullet should be. Let’s take a look at the 240 grain MatchKing. This “mamba-jamba†freight train of a bullet shares the same tangent ogive nose design as all of the .30 caliber MatchKings down to 155 grains. The ogive looks short and blunt on such a long bullet which affects the aerodynamics which will be the topic of the next section. The long bullet has other common problems as well, in particular, the excessive copper fouling caused by the long bearing surface. Prolonged success with the 240 grain MatchKing is intermittent at best, and experiences vary among those who’ve tried them in competition. At 11 grains over the trendline, maybe it’s just a little too long.
The 220 grain MatchKing is only 9 grains below the trendline, which isn’t so bad. As far as legitimate “heavyweight†.30 caliber bullets, this is probably the best option available, yet it doesn’t seem to be very popular.
I think part of the reason these heavy .30 cal bullets get overlooked is because many shooters think that a 185-190 grain bullet is “heavy†for .30 caliber. In fact, a 190 grain .30 caliber bullet is somewhat of a “middleweightâ€. To put it in technical perspective, a .30 caliber 190 grain bullet is proportional to a 168 grain 7mm bullet, 120 grain 6.5mm bullet, or a 95 grain 6mm bullet. The 155 grain bullet used in Palma competition is very much a “lightweight†for .30 caliber. 155 grain bullets are used for international Palma competition because the rules specifically require it, not because 155 grains is the best weight for a .30 cal bullet at long range. Recently, some 155 grain bullets made by Berger and Sierra are designed with different, more aerodynamic profiles that help to compensate for being so light. The reduced drag helps them make up some ground compared to their conventional heavier counterparts,
