A discussion on Sectional Density and Ballistic Coefficient. This is an excerpt from my video on Bullet Selection for those of you who just want to review SD and BC. Experience Level: Advanced Experience Level: Intermediate
Gravity is a constant. Higher velocity doesn’t change the effect of gravity at all, the bullet trajectory is flatter simply because it is covering a farther distance in a shorter amount of time.
Not quite. Depending on your latitude, the direction you're shooting, and the velocity of the bullet, centripetal acceleration of the bullet about the Earth's axis of rotation will slightly change the rate at which it falls to the ground. If you're shooting either due North or due South, then the bullet will fall at exactly the same rate as if it were dropped from rest. However, if shooting due East, the bullet will fall to the ground slower than if dropped from rest. When shooting due West, the bullet could fall slower than, identically to, or faster than if dropped from rest, depending on how the velocity of the bullet compares to the local tangential velocity of the Earth.
I appreciate this explanation. Been reloading 2yrs and still feel like an infant. Tutorials like yours make it possible for me to develop as a reloader and a shooter. Thank you sir. Subscribed
Thank You for explaining this. I really didn't understand how BC equated to numbers until now. I knew what it was but how it translated to 0.329 as an example was a mystery until now. Thankyou also for making it such a thorough presentation. I feel considering what you've covered 30 mins was well needed. Respect from Australia
All in all, this is an excellent explanation of these two parameters that are, I think, not very well understood, but which are very important in selecting a bullet based on its expected performance. I have used BC on various ballistic calculators and also to calibrate the markings on a ballistic scope reticle. In my case, it was Nikon's Spot On program. I find it maddening, when the manufacturer does not include BC data on the box, as was the case for a Remington 180 gr PSP I am using for an elk hunt. With respect to sectional density, I very much appreciate the chart showing various sectional densities for different hunting objectives. I now know why the bullet I used last year with an SD of .210 was not a good choice for elk, where you recommend a minimum of .230. I have just a nit on the slide "Calculating Sectional Density for Bullets." The final formula is correct in that you need to multiply the denominator by 7000 to convert lbs to grains of bullet weight. The intermediate step showing both numerator and denominator being multiplied by 7000 is not correct, mathematically, as that would be the same as multiplying the entire equation by 1. It would be better just to say convert lbs to grains by dividing by 7000. A final comment on the last slide, where you talk about making the assumption that a higher BC gives better performance. The caveat, which you do say, is for the same muzzle velocity. Otherwise, as you point out, a lower BC bullet may outperform one with a higher BC, if it is loaded to yield a higher muzzle velocity. I am assuming, therefore, that when manufacturers calculate BC, it is for an assumed loading that yields the design muzzle velocity. Another great video. Keep up the good work.
+J Heimer Thanks J Heimer, they are factors that many reloaders either disregard or don't seem to understand entirely. The sectional densities I list for various game are based on my independent research as well as many hours of discussing the subject with hunters for whom I have a lot of respect regarding their knowledge of the sport. I've learned quite a bit from them over the years. As far as the math, well, I wasn't kidding when I said I slept through most of Mrs. Younkin's 10th grade Algebra class, lol. I'm not an expert in math by even the lowest standards. I went through several iterations of how to describe the conversion, and this one seemed to illustrate best why the final equation had the diameter multiplied by 7000. I have no doubt that it wouldn't pass muster with someone who has a more intimate understanding of mathematics. This one just seemed to be the clearest way to describe it for those of us who still need a calculator to add up our grocery bill, hehe. Manufacturers do tend to calculate their BC based on the muzzle velocities that their load data manuals dictate. Sierra does have a good section on Ballistics Coefficients in their manual and they do list multiple BC's for their bullets based on Muzzle Velocity. Hornady and some of the others seem to ignore the fact that velocity has an impact on the bullet's performance. Thanks for watching and for the help with the math!
Boom...Mind blown lol...... But It really did help me understand it much more! Thank You!!! and thanks for taking the time to make all your vids for us! So helpful! Rich
Very helpful! (I think on your Using Section Density slide at 11:00, the "antelope" should be on the Small Game row, since an adult male typically weighs less than 150 pounds.)
Almost every source I find uses diameter squared as the "cross-sectional area" of a bullet. Being that bullets have circular cross-sections, is it purely historical convention that we don't use pi times radius squared? (Which would be the true cross-sectional area of a circle). Thanks in advance!
+taiming71 Thanks taiming71, It's an important aspect of reloading and bullet/cartridge selection so I thought it was important to put up a video on it for those folks who may not have been familiar with the terms. Thanks for watching!
So the longer bullet with a higher BC and SD is better than a bullet with a lower bc and sd....if they're traveling at the same speed. So a 3006 shooting the 168 gr bullet might be deadlier and more accurate than a 308 shooting a 155, because the 308 has about 10 gr less powder.
I think folks confuse the facts and what BC and SD mean when it comes to "Devastation" and "Leathality". If you want to say all that matters is gravity, you need to understand there is more than gravity, there is also resistance of the air pressure. Higher BC will be more aerodynamic and have less resistance to flight and go further than the same weight bullet, with the same energy but higher BC. If a 10mm will kill bear, which it will, just like a .22 cal will kill a large hog, if you to a brain shot or interrupt the central nervous system. The reason 10mm is now a trend in Alaska for bear defense is due to it will penetrate the bears skull, it offers capacity, so follow-up shots and most importantly it is easy to carry when you also have a fishing pole in your hand. If you didn't bring it with you, you left it in your truck, you cannot use it to stop a bear when it isn't in your hand. The cartridge size or amount of potential energy, is the delivery system. If your target is several hundred yards away and has a tough thick body, like a moose, you need the energy to accurately deliver the bullet to the moose and then still be able to punch through it. At 2200 fps a bullet also does shock-wave damage that effectively adds a significant effect to bleed-out and leathality. If you have a large hog or bear, and that large predator is charging you in the thick brush, you would want a bullet with the ability to punch through the brush and then punch through the bears skull, and better yet the ability to have a lot of fast follow-up shots to assure you score a critical hit and do a lot of devastation. A lot of wild animals can be heart shot and run 100 yards, live another minute. If you really want to drop something in its tracks you need to do a Central Nervous System shot. If you are just hunting game and trying to save meat, you tend to go for a heart shot, with intent to cause a large wound channel all the way through the animal so that they bleed out ASAP. There is no magic bullet; but the bigger wound channel, even on a large predator with a low heart rate and lot of fat to help clog the wound channel, can still take a minute to bleed out. A blown out brain tends to drop them in their tracks, so really all the big caliber high energy rounds only amount to getting the bullet to and through the target. Otherwise you only need something big enough to blow out the brain and switch off the CNS. Sometimes you miss the precise placement though, so going for the heart shot with a massive trauma causing bullet to center of mass will still cause a lethal bleed, if you still hit the chest and miss the heart. SD is about punch, BC is about not getting blown around in the wind and slipping through the air easier. Slipping through the air also means slipping through the target, unless there is expansion and change in BC. Energy beyond the other side of the target is wasted, unless you are trying to hit a second target too with the same bullet.
wrong to a point,that is what the bullet is made of and how it is made, soft hard bonded ect,so sec des of .240 is better for deer,.270 ELK, .300 dangerous game, so whats it made out of copper/lead/ or cheese, hardness values.
