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Discussion in 'The Basics, Starting Out' started by taxidermynut, Mar 22, 2006.
I just got a box of 175 gr. uld's and was wondering if anyone has an estimated BC is for them.
Berger bullets lists the BC of their VLD's. I have found that simular weight Wildcats must have nearly identicle BC's. So I've used that to get me in the ball park. Then of course chrono testing and range time will give you the true effective BC.
They don't make a 175 7mm But i would imagine that it's BC might be in the .680 or so
Fiftydriver might have some insight on this. If he dosen't see this thread soon shoot him an email.
Sierra 175 gr MK have a BC of about 0.60. This would be a good place to start from on a trajectory calculation and then as was suggested make your own corrections as always.
I would say start with at least a BC of 0.650 and figure it up from there. The 169.5 gr ULD RBBT in 270 cal has a BC(derived from bullet drop testing out to 1000 yards) of 0.740.
It would not suprise me to see the same BC with the 175 gr ULD RBBT in 7mm or at least very similiar. Remember this is derived from bullet drop tests out to 1K, not from velocity comparision over long range so those ready to pound on the keyboards, go get some coffee and relax /ubbthreads/images/graemlins/grin.gif!!
Any of the BCs offered will get you close at moderate ranges, then its time for you to do some work and figure the exact number for your unique situation.
From reading on this site I think I understand how to calculate it, shooting at different distances through the chrono. Is there a way to get a close enough BC to be accurate out to 800yds, and not shot through the chrono past say 300 yds? Don't know that I am comfortable with distances farther than that, as I'm on a second chrono already. Thanks for the help.
Correct me if I'm wrong. you should only have to check muzzle velocity. Your down range velocity will change as your BC changes. Zero your rifle at whatever range, use that number (zero range)for your ballistic program, measure your drop in 100 or 200 yard increments out to your maximum range, then play with the BC number until your drop numbers (low of zero) match. Or do the opposite, zero for 800 and work backwards.
What you can do is shoot through a chronr so you definantly know what your intial velocity is. After that use a ball park BC and shot the gun out to 500yds and figure up how many clicks of elevation you must put in to be able to hit your target. Continue doing this out to as far as you can. If you can get out to 1000yds or more then you will be good.
After this go back home and get on the puter and find any of the ballistic programs that are out there. Use you velocity and ball park BC to plot a trajectory out to your farthest shot. AFter this simply tweak the BC until your trajectory matches the adjustments you had to make to be able to hit your target at the various distances. Remember to put in all applicable info from that day that you can into the ballistic program, will help you to be more accurate.
This may not be the "textbook perfect" way to figure a BC but it works and most importantly you will figure out a drop chart and be out shooting.
Most of us that have been on LRH for a while have been through enough to these discussions on the proper way to figure BC to know everyone has their own way of doing it.
TO get a scientifically pure BC you would need velocity measurements over the entire bullet flight. Only way to really do this is with a dopler radar system. Most of us do not have one of these in our back yard so we have to figure something else.
I will not go into it deeply because it has already been mentioned by the previous posters.
All I will say is that where the actual bullet is in relation to your reticle at a known range is really the only important thing we as long range shooters need to know. Having a scientifically pure BC value is not needed to figure this out.
Predicting, testing and proving a certain loads trajectory is the only accurate way to do this. Then using ballistic tools such as ballistic programs and such can adjust for changes in different shooting conditions.
Getting a ballistic programs trajectory model to match up as perfectly as possible with your actual bullet path is the key. No matter how you do it, it has to be done.
You can do a search on this topic and have more reading then you will ever want to read, some more useful then others!!
I you guys don't mind me chiming in here...
Pure BC is not all that important as the BC calculated from drops. Even though it may not be the TEXTBOOK method as Lerch put it, it works. Sierra is the only company that posts BC changes with velocity changes. Even that is not 100% accurate as BC continually changes as does velocity as the bullet travels.
If shoot a 200 SMK in Maine then shoot the same gun with the same ammo in Arizona, my velocity changes and so will my BC along the way.
Bottom line is that the BC that counts is the BC that matched you ballistic programs with your drop chart where you shoot. As long it is a high enough BC for your long range work. 680 or 750, or 900 doesn't matter.
I like to chrono the mv then shoot different ranges and then plug in BC numbers until my program matches my actual drops.
Remember, nothing beat shooting in the real world.
Discussions like this are the reason I enjoy to this board. Everyone learns something new, or a new way of looking at a problem that may give you more insight into the root of our beloved ballistic charts.
Being the eternal perfectionist there are a few factors that make it nearly impossible to 100% calculate the bc of your bullets. Two guys with two different guns shooting the same bullet at the same muzzle velocity can get very different bc's because of a few things we normally don't think of and take for granted.
1. We all accept that our scopes have exactly 1/4 moa clicks, now what if it's not actually 1/4? or even worse, a gradient scale because of different manufacturers internal mechanics or manufacturing inconsistency. One shooters scope has a slightly less than 1/4" travel in a click and another has a little more. The two shooters would come up with different bc values as a direct function of this condition. I consider this to be the biggest possible reason for mathematical variations between field calculated BC's.
2. Wind changes value and direction down range because of topography and makes it tough to get an exact drop value even as an average.
3. Temperature changes as we test, causing the air to beacome more or less dense as we run our test. this will also affect the outcome slightly unless you can find a sweet spot in the middle of the day that allows you a window of constant temperature.
4. The last variable to consider is roll-over of the bullet at extreme ranges. this can cause the bullet to slow considerably more if not perfectly weighted. When you let a paper airplane fly, if it is properly weighted it will nose down and fly towards its target in a perfect arch. if not enough weight is in the front the plane will stall as velocity falls and the wings act as a parachute and not a wing. too much weight in the front and it will nose down too early and seriously drop the range of the plane. This is an extreme example of roll over on paper airplanes but will help explain my next statement. If a bullet is perfectly weighted it will roll over to match its direction as it travels downrange. In this perfect situation the air sees the same profile at all times. If its not properly weighted the bullet will prematurely roll, or not roll at all as its direction changes in its natural arch. This causes the bullet to increase its profile and its bc will seemingly magically decrease.
My reasoning behind this post is to convey the fact that each rifle/scope combo should be tested to find it's exact trajectory path. Use the bc as a tool to find your trajectory path as close as you can measure it in field testing. Dont trust the bc as an end all be all unless you have used the very same rifle at the very same altitude to obtain the approximate bc. There are too many non-finite factors contributing to the calculated bc that can cause variations in drop values that can baffle a person. I stopped making drop cards before hunting trips because the range cards I make on site are far more valuable and accurate.
Yes, I'm Anal and I know it. This may answer a few questions for people who have had problems calculating a constant bc to match their charts. Anybody have a aspirin? My head hurts!
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680 or 750, or 900 doesn't matter.
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While I agree with your sentiment for the most part, not quite to those extremes. That's a big spread. It will matter when the weather changes or you go to a different elevation. It will matter for wind drift even more than drop.
My best advice is to get the best BC number you can by eliminating as many other variables as you can for drop tests or better yet, measuring the BC as accurately as possible.