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Ballistic Coefficients
needforspeed
Member Posts: 48 ✭✭
I'am new to the sport of reloading for target shooting, I've had plenty of experience of working up hunting loads and have produced some good ones. One for example is for my deer rifle I worked up a load and shot a five shot grouping that measured .700 at 100yds. I've started reloading for accuraccy since I bought I 12Fv Savage. As I started reading I came across Ballistic Coefficient(BC). The only real diffention that I have got was In the Seirra Manual it says "A mathematical expression of a bullets ability to overcome atmospheric resistance(drag)" In the manual they also have a BC Table, giving the BC for all of there bullets in all calibers at different velocity's. Know what would be better a 168 grain in .308 with a .447 BC or a 175 gr. with a .496 BC.What is better the higher the number or the lower. Thanks
Comments
Like 37 rangemaster said and to quote Sierra, "The higher the better to overcome resistance". What you need to do is crunch out some numbers. What you will generally find is that out to 300 yds. BC doesn't mean a whole lot. It means a lot to the ultra-competion crowd who are measuring as small a group as possible. And, which at 300 yds. could mean a 1/4 or 1/2 and inch for a group it will make a difference. but to the hunter it doesn't mean a tremendous amount until you get past 300 or so yards.
Where ballistic coefficient really makes a difference is the 1000 yd. shooting. Where .2 better co-efficient can mean as much as 100 inches(velocity plays in here also) of difference in how much a bullet drops. So from about 300 yds on the BC just gets more and more important.
The closest I can come to comparing apples to apples is to run a program (JBM trajectory) to show the differences. I used a 180 gr. 7mm Berger VLD with a BC of .699 and Sierra's well known .308 168gr HPBT. Both fired at 2600 fps. standard seal level pressure and temp.
The Berger drops 336 inches @1000 yds. and drifts 65 inches in a 10 mph wind.
The Sierra drops 465 inches and drifts 124" in the same wind.
Reverse the bullet weights and use a 7mm(.284) Berger 168 gr.(BC= .649) @2600 fps. std cond. and the drop is 349" and 72" of windage with a 10 mph wind. The .308 Sierra HPBT 180 gr. bullet (BC=.493)travelling the same speed drops 416" and drifts 103" in a ten mile an hour wind. So you can see how important BC is in the long run. -good luck with this project.
Better is a relitive term, because if your rifle dosn't like the paticular bullet, the fact that it has a higher BC won't mean squat. IOW if your rifle shoots 1 hole groups with a 200gr RN but won't group a SMK on the same piece of paper, does the higher BC help your deer hunting?
In general (the manual explanation is at "BC for dummies" level) the higher the BC the lower the wind resistance, and as a result the bullet drops less and resists wind drift better, than one with a lower BC will.
Published BC is a ratio, with 1.00 matching the "standard" G1 (1" dia, solid iron, flat base with a 2r radius nose) projectles wind drag numbers.
Reloading for target shooting is just a more highly refined version of reloading for hunting. The basics of reloading are expanded and more detail is added in order to try and qualify all of the variables that have an affect on accuracy.
The Ballistic Coefficient is one small part of process and generally only comes in play at longer yardages. My target loads for the .38 Special uses a 158 gr. wadcutter with a BC of .121 and is superbly accurate if I'm up to it. My Palma rifle uses a bullet with a BC .442 and is also very accurate. Big difference between the BC's but both systems are accurate.
Rather than write a dissertation on ballistics, I have supplied some links for you to read and print out if you want. Be sure to understand that the accuracy you may be looking for is made up of dozens of variables and that there are opinions that vary across the full spectrum of for and against in each situation. You will also find that the more information and opinions that you try to integrate into a concept of accuracy, the more confused you may become. Don't worry, just start the ball rolling and go shoot some more. Keep good records for both your loads as well as each shot that you fire. Then spend some time analyzing the information afterwards. Without records and thought, you'll just be shooting for fun.
Use my contact information if you need some more to read or have questions.
Best.
This is one of the better short explanations. Be sure to look at the rest of the list of technical articles since they can be a big help in developing accurate loads:
http://www.shootingsoftware.com/coefficients.htm
http://www.shootingsoftware.com/tech.htm
This is a good introduction article:
http://www.loadammo.com/Topics/September01.htm
This is a link to good article on Ballistic Coefficients and how to figure them:
http://www.uslink.net/~tom1/calcbc/calcbc.htm
This is the index link for his website. There is some good reading here:
http://www.uslink.net/~tom1/index.htm#Top1
I had typed up a longer response and somehow lost it while posting.
Does the component parts and content of a particular bullet make a diff?
I have an .06 that loves 180 grain bullets. I use to have an .06 that loved 200 grain bullets. Both had the same rate of twist.
SD is simply the relationship between diameter and weight, all 30cal 180gr bullets have the same SD.
Twist rate effects stability (bullet wobble, going through target sideways, keyholing etc) not so much the accuracy (small groups).
It's bullet length, NOT weight that controls the required twist rate, that is to say that a 180gr ULD design would require a much faster twist rate than a 180gr roundnose (BTW both have the same SD).
What your seeing with your 2 -06's is more related to barrel harmonics and what each rifle prefers.
BTW, my -06 likes 150's in general, and loves the 150gr NBT. The heavier I go the larger the groups I get (even though the holes are all round, bullets are stabilized).
Each rifle is a law unto it's self as far as the bullet/load it prefers.