Understanding Long Range Bullets Part 2
Understanding Long Range Bullets Part 2: Practical Considerations and Decision MakingBy Bryan Litz
Last month, we identified some basic trends that occur when you scale bullets up and down in caliber. Most of our findings were fundamental and academic. This month, I want to take a close look at some of the practical consequences of scale. Whereas last months article focused on establishing facts, this part will give some conclusions, judgments and some of my opinions based on the facts.
Caliber and bullet choice are two very important considerations that a shooter has to make regarding his equipment. Too often, important choices like these are made for the wrong reasons and shooters end up disappointed in the performance of their equipment. Choosing components based on how well they work for others can be a bad idea. Every shooter has unique objectives, preferences, abilities and budget. In this article, Iíll attempt to present some information that new shooters can use to make informed decisions about what equipment is best for them based on the consequences of scale.
Defining ĎSuperior Ballistic Performanceí
When it comes to ballistic performance and long range target shooting, there are things that are important, and things that are not. For example, some people may compare trajectories, and conclude that a Ďflatterí trajectory is better for long range target shooting. Well, itís not. The reason that drop is not important to target shooting is because all of the targets we shoot at are at known distances! A flat trajectory is important if youíre unsure how far the target is, and you want to minimize your vertical miss distance due to errors in range judgment.
So what is an important Ďmeasure of merití? When it comes to long range target shooting, wind deflection is the most important factor of external ballistic performance. The caliber/bullet/velocity combination that is least affected by wind deflection will stand the highest chance of consistently shooting the smallest group and the highest score, period. There are many other factors like consistent muzzle velocity, seating depth, leade angle, optics, quality of components, etc. All these things are important factors, but theyíre not external ballistics. More importantly, all of these things are Ďdeterministicí, meaning that once you figure them out, youíre good to go. A load thatís good this year will be just as good next year, provided nothing else changes. The wind, however, is not deterministic. The wind blows different every day, and for every shot. Having good external ballistics can help you to better cope with the indeterminate wind that plagues long range shooting.
I know that wind is not the only consideration; weíll get to others later on. Right now, I want to show you something about wind deflection.
Table 1 illustrates some trends regarding the .224 thru .30 caliber bullets that we analyzed last month. Remember, the 6.5 mm 142 grain SMK at 2950 fps is the Ďbenchmarkí to which the other bullets are compared. I chose this caliber, bullet and velocity because of its popularity with long-range shooters. A velocity of 2950 fps is typical for the popular 6.5-284 cartridge which is often used with the 142 gr bullets.
What you should notice right away about Table 1 is the fact that the larger calibers require a lower muzzle velocity in order to match the ballistic performance (wind deflection) of the benchmark, and the smaller calibers require more velocity. That shouldnít be a surprise. So whatís Table 1 telling us that we donít already know? Well, first thing is that itís nice to know the actual velocities that are required to match the benchmark for other calibers and bullets. From those velocities, itís possible to compare the recoil of other calibers to the benchmark, and get an idea of barrel wear. Weíll talk in more detail about recoil and barrel life in the next section. Right now, letís continue the discussion of wind deflection.
Understanding Wind Deflection
One way to think about wind deflection is in terms of ílag timeí. Lag time is the difference between the vacuum time of flight3, and the actual time of flight. For example, say you shoot at a target 1000 yards away with a bullet of initial speed 3000 fps. The vacuum time of flight is 1.00 second (3000 feet at 3000 feet per second). Due to atmospheric drag slowing the bullet down, the actual time of flight may be closer to 1.6 seconds. In this case, the lag time is 0.6 seconds. From here, calculating the wind deflection is easy. Just multiply the lag time by 54fps) crosswind for the 1000 yards of bullet flight. 14.66 fps X 0.6 seconds = 8.8 ft, or 105 inches. The hard part in all of this is figuring out the actual lag time. It depends on actual time of flight, which depends on BC.
Bullets with higher BCís will always have less lag time when fired at the same velocity. The question is, how much velocity can you give up with a higher BC bullet, and still have less lag time? We can get a rough idea about this from looking at Table 1. As a general rule, you can go about 496 fps slower for every +0.100 counts of BC, and match wind deflection. For example, our benchmark142 gr bullet going 2950 fps (BC = .565) has 70 inches of wind deflection in a 10 mph crosswind at 1000 yards. What speed does a heavier bullet having a BC of 0.5964 need to match the wind drift of the benchmark? Well, thereís 0.031 difference between the BCís, so .31 x 496 fps = 154 fps. So a bullet with a BC of 0.596 only needs to have a muzzle velocity of 2950 fps . 154 fps = 2796 fps in order to match the wind deflection of the lighter faster benchmark. This compares well with the 2800 fps in Table 1.
To echo last month: Most of the calculations in these articles are not intended to show hair splitting precision, but rather to illuminate some basic trends, and approximate magnitudes. Having an educated intuition and some useful rules of thumb can be helpful when youíre trying to make decisions involving so many complex variables. In other words, donít hold me to it; itís just an estimate.
BC Advantage of the Larger Calibers
In Table 1, Iíve shown the velocities required for each bullet to match the wind deflection of the benchmark. What may not be clear is that it is easier to achieve those velocities with the larger calibers than the small ones. For example, the little 90 gr .224 bullet has to get to 3270 fps. Thatís not easy. Excessive pressure, accelerated barrel wear, and possible bullet failure happen at this kind of speed. The 2950 fps of the benchmark is a Ďstoutí velocity for the 6.5-284. 2800 fps in the 7mm can be done with the same case and powder as the 6.5-284. I donít want to get into a discussion about case volume and powder efficiency here because Iím frankly not that knowledgeable about it. Besides, this is about external ballistics, not internal. What I can say is that itís easier to achieve 2650 fps with a number of .30 caliber chamberings and 220 grain bullets than it is to achieve 3270 fps with .224 chamberings. That means that there is more potential to beat the benchmark 70. of wind drift by going to a larger caliber, rather than smaller.
Of course, driving the heavy bullets at higher speeds results in less wind deflection, but you may run into the same problems that the smaller calibers have like high pressure and short barrel life. Not to mention the crippling recoil! Fast heavy bullets are best suited for heavy benchrest rifles that can absorb the recoil. I know only a few people who can successfully manage the recoil of a 30-338 prone rifle. Those who handle the recoil definitely have an edge in the wind. Be honest with yourself about your recoil comfort level before you decide to go with such a monster. More on recoil considerations later.
BC Isnít Everything. or is it?
Iíve made a big deal out of BC so far. Iíve claimed that itís the most
important measure of merit for long range ballistic performance, and that itís
easier to achieve overall better ballistic performance with the larger calibers.
However, before we leave the subject, Iíd like to make an important point. BCís by themselves donít win matches! You canít show up at a match with what you believe to be a superior ballistic combination of caliber and bullet, and expect your equipment to win for you. The difference in ballistic performance among long range rifles at any given match is relatively small. Only a couple percent separates the best from the worst ballistic performer. Your ability to effectively read the wind and properly adapt to the subtle changes are far more important than a few points of BC. However, high BC is still important.
Consider the following situation. Two competitors shoot a match against each other in the same conditions. Shooter A has a small (5%) ballistic advantage over shooter B. If both shooters are equally skilled at reading the wind, shooter A stands a slightly better chance of winning that match. If these two shot 20 matches against each other, Shooter A is almost certain to out Ďaggí shooter B (have a smaller average group size). The more the wind blows, the more advantage shooter A has.
1Average advertised BC for 2000 fps thru 2850 fps from last month's article.
2Wind deflection is for a 10 mph crosswind at 1000 yards.
3Vacuum time of flight is the time it would take the bullet to get to the target if it maintained its original muzzle velocity all the way.
4Average BC of the 7mm 175 gr bullet from 2000 fps to 2850 fps.
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