For all of you out there that have ever fallen asleep during a discussion of external ballistics, this is not the post for you. Unless you are suffering from insomnia, and desperately need to get some hard core rest in the next 30 seconds, I suggest that you move on. Those that didn't heed my warning and are still reading, now I can talk openly with you about ballistic co-efficients, windage, and foot pounds of energy without running the risk of scorn. Doesn't it feel great that we can communicate on this deeply personal level?
Okay, down to brass tacks. It turns out that the .308 using 168 gr. Sierra BTHP Match bullets shot from 2650-2750 fps will experience a bullet drop at 1000 yards of about 386 inches. The ballistic co-efficient (hear after denoted as BC) of that round is .462-.424, depending on the velocity. The .243 105 grain Hornady A-Max bullets I have, boast a general BC of .500. I looked up the BC for the Sierra 107 gr. BTHP Match bullets and they fall in at .527-.495, (velocity dependent, from 2900 fps at the muzzle). This higher BC means that the .243 will only experience a 309 inch drop in 1000 yards. Not only does the .243 round out perform the .308 in the amount of bullet drop, it gets there faster. Time of flight (here after denoted as ToF) for the .243 is 1.495 seconds, while the heavier, slower .308 takes 1.688 seconds to impact the target. This flatter trajectory means that the .243 round will be more forgiving with regards to mistakes in the range estimation of any given target, than the .308 round will.
The particular .243 round we are discussing also out performs the .308 round with respect to wind drift. If we examine a 10 mph wind coming at 90 degrees, the .308 round will drift 105.1 inches. The smaller, faster .243 will only drift 81.1 inches. I suspect this is due in part to the shorter ToF.
The one area where the .308 will out shine the .243 is the amount of energy the round has. Assuming you can get your 168 gr .308 round to 2750 fps at the muzzle, you will have 2821 ft/lbs of energy. The 105 gr. .243 round at 2900 fps, will give you 1961 ft/lbs of energy. That means the .308 has 860 more ft/lbs of energy at the muzzle. However, lets take a look at the 500 yard, and 1000 yard markers for a closer look at how each round bleeds energy. At 500 yards, the .308 is down to 1271 ft/lbs. That means that it has only retained 45 percent of it's energy. The .243 round is down to 998 ft/lbs at 500 yards. It has retained just shy of 51 percent of its energy. At 1000 yards, the .308 round is down to 534 ft/lbs. It has only retained 18.9 percent of the total energy it once had. The .243 is down to 463 ft/lbs at 1000 yards, meaning that is has retained 23.6 percent of its total energy. So although we cannot argue that the .308 round has more energy at all ranges, it is interesting to note that it bleeds off energy faster than the .243. For the purposes of this discussion, I say that makes the .243 a more efficient cartridge with respect to external ballistics.
If you are still awake after reading this, I congratulate you on the tremendous feat you have just accomplished. I suspect, that it was much harder for you to read this and remain coherent, than it was for me to write it. I hope you learned something, because I sure have. The .243 is no slouch, ballisticly speaking.
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