Understanding Long Range Bullets Part 2

By ADMIN · May 25, 2015 ·
  1. ADMIN

    Understanding Long Range Bullets Part 2

    Practical Consequences of Stability
    Last month, we noted that the larger bullets require slower twists to be stable, and explained why. Now for the important question: Is there any practical consequence to a faster or slower twist? I can think of a couple things here. The first one has to do with the structural integrity of the bullets. It’s believed by many shooters that high twist rates combined with high muzzle velocities are responsible for bullets breaking apart in flight. Many other factors play into the bullet failure problem like roughness and length of the bore, sharpness of the riflings, etc. It seems that people with the most bullet failure problems are shooting small bullets at high speeds, thru fast twist barrels.

    The second consequence of high twist rate has to do with accuracy ‘potential’. Some major factors affecting dispersion are related to the spin rate of the bullet. Jacket eccentricity and in-bore yaw are two examples. These types of dispersion are directly related to the RPM of the bullets, which increases with twist rate and velocity. Twist and velocity are two things you need a lot of to be competitive with smaller calibers, and can exacerbate certain types of dispersion.

    Keep in mind these are only ‘potential’ consequences of excessive twist. Many shooters do very good shooting long, small caliber bullets at high velocity without having bullet failure problems, and with amazing accuracy. However, it’s something to consider.

    To be fair, there are similar ‘potential’ accuracy robbing effects with larger calibers like excessive recoil on not just you, but on your rifle. There’s a video online that shows someone shooting a .50 cal rifle in slow motion. During recoil, you can see the scope wiggling like a noodle! I know most of us don’t shoot .50 calibers, but that video drove home the point that you better bolt things down hard if you’re dealing with a lot of recoil. Excessive recoil is more likely to mess with your optics and bedding. Also, barrel whip and muzzle blast are potential accuracy robbing side effects of the larger calibers.

    One last point regarding stability: Starting as soon as the bullet leaves the barrel, gyroscopic stability begins to increase. The reason is simply because the forward motion of the bullet is decreasing much faster than the rotational speed of the bullet. In other words, de-stabilizing effects are going away quicker than stabilizing effects. The result is that Sg grows as the bullet goes downrange. Now wait a minute, why is it that some bullets are known to tumble at long range? Well, everything changes when you enter the transonic speed regime. Aerodynamic coefficients spike, unsteady shocks and transient effects are acting to erode the dynamic stability (Sd) of the bullet, which is different from the gyroscopic stability (Sg) that we’ve been talking about. I’ll save transonic stability for another article.

    For now, just remember that a safe rule of thumb is to insure that your projectiles are fired at a high enough muzzle velocity that they will remain comfortably supersonic (greater than 1120 fps) all the way to the intended target. This way, you will always insure stability.

    Up until this point, we’ve been talking mostly about the external ballistic effects of scaling bullets to different weights and calibers. Of course, there are other things to consider when you decide what caliber and bullet will be best for you.

    Comparing Recoil
    Recoil is an important consideration that many shooters probably wish they had made more carefully. Sometimes the superior ballistics of the big magnums and high BC bullets are so compelling that folks don’t think about the beating that they’re in for! Make sure you know how much recoil you are comfortable with before you decide on a caliber and bullet. The flinching caused by heavy recoil may do more damage to your group or score than the little bit you gain from better ballistics.

    Table 1 shows the recoil energy, in foot-pounds, for a 16-pound rifle. The recoil was calculated using the method given in Ref 1, where the recoil energy of the powder is accounted for as well as the bullet itself. Table 1 is meant to show the recoil generated by the different caliber/bullet/velocity combinations required to match the wind deflection of the 6.5mm benchmark.

    Once again, it’s important to understand the trend here, more than the actual numbers. If I told you my rifle had .X. foot-pounds of recoil, that wouldn’t mean much to most people. But Table 1 compares the numbers, which allows you to see how much the recoil is different for the various calibers and bullets. For example, you can say that the 7mm, 175 gr bullet has 18% more recoil than the 6.5mm benchmark5. When recoil is compared to a known value, it actually means something.

    Of course, you may choose to offset recoil energy by shooting a heavier rifle. If you wanted to shoot the .308 220 gr bullets at the speed required to match the wind deflection of the benchmark, you would need a 23.2-pound rifle to also match the felt recoil of the benchmark. Matching recoil with the 7mm 175 gr bullet requires an 18.9 pound rifle. The 6mm could weigh as little as 13.5 pounds and match the recoil of the 16.0 pound benchmark, and the .224 caliber rifle would weigh 10.3 pounds.

    Comparing Barrel Life
    Barrel life is one of those things that can’t be nailed down because it’s different in every case. Also, there are many factors involved that can help or hurt the useful barrel life you get out of a given rifle. For this reason, I chose words instead of numbers to describe the barrel life in Table 1, just to show the trend.

    The caliber, bullet, powder charge and resulting velocity are the most important factors that determine barrel life. However, there are things you can do to promote longer barrel life like proper cleaning and using moly-coated bullets. Excessive or improper cleaning can decrease barrel life.

    You should think about the following things when considering how important barrel life is to you.

    1. How often do you shoot, and what types of matches? If you’re a casual benchrest shooter who makes it out to 5 or 6 matches a year to fire 30-40 rounds per match plus a few practices, it may take several years to wear out even a short life barrel. However, if you’re a long range prone .junkie. who attends 3 or 4 matches a month, sometimes firing over 150 rounds in a single weekend, you will wear your barrel out much faster, possibly in less that 1 season.

    2. How deep are your pockets? A new barrel chambered and installed runs about $500.

    3. How much do you want to bother with the inconvenience of packaging up your rifle and sending it away to be re-barreled? The tragedy of having a barrel go out in the middle of the season can lead to desperate times. Trying to find a gunsmith who has a barrel and time to chamber and install it for you in a rush isn’t easy. Even if you have a back-up barrel ready to go, you need to send your rifle away to have it installed, unless you have the tools and know-how to do it yourself.

    4. How high are your standards for accuracy? This is important because some barrels wear out slowly, and you may get several hundred rounds of ‘acceptable’ accuracy after the peak accuracy of the barrel is gone. Serious shooters often change out their barrel as soon as they suspect that accuracy is starting to go downhill, whereas casual shooters may finish out the season with a barrel that’s past it’s prime. Of course, some barrels stop shooting all at once, within less than 20 rounds.

    In my opinion, the uncertainty involved in barrel wear is a big problem. Barrels that wear out quickly make it hard to plan. Even if you keep an exact record of how many rounds you.ve fired, the barrel may wear out sooner than expected.

    Another thing that’s annoying about barrels that wear out quickly is that the lands wear so fast that you’re constantly seating the bullets out longer. Each match is like an experiment and you may never settle on a consistent seating depth for the whole life of the barrel. Of course, soft seating the bullets into the lands helps with this problem.

    Having said all that, there is some recent activity involving small caliber (6mm) cartridges that have case capacities and shoulder angles carefully designed to be gentle on barrels, and wear them out slower. I haven’t kept up with the onslaught of new 6mm case designs, but it sounds like they.re making some progress regarding barrel life and combustion efficiency.

    Remember, it is possible to achieve very long barrel life with a smaller caliber at reduced velocity. If you want to keep up with the ballistic performance of the 6.5mm benchmark, your .224 or 6mm barrel is gonna cook!

    5When loaded to match the wind deflection of the benchmark.

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