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In particular I,m presently interested in the true BC of the Sierra 69 TMK (224),,,,on the Sierra product info it gives BC,s at differerent velocity windows .Now,,,are we to take the top BC figure as long as our rifle is launching at this speed or should we be taking an average of all the BC,s quoted. I am asking as in reading on another forum where some testing had been done on this bullet and others that they quote a figure way down on the highest figure quoted by Sierra,,,,ie .345/355 as to .375 in the Sierra product info.I hope I,m explaining this well enough in order for you to see where I,m coming from but I have always used top quoted BC figure given by manufactures and plugged in to Ballistic Apps and has seemed to work albeit with some fettling from time to time,,,,,would an averaged BC work better ??after all it seems that BC is continually decreasing over velocity range,,,,do ballistic apps take this into account??? your thoughts,,,learned gentlemen please,,,,thanks ,,O

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Drag coefficients always change with velocity (see picture below). What the Gavre drag function (G1, G7 etc.) does is to mathmatically relate a particular bullet to the drag coefficient of a 'standard' bullet shape. These were originally calculated from firing trials conducted in the late 1800s.

 

BCs are always a mathmatical fudge and the modern method for military projectiles is to calculate the actual drag coefficient using doppler radars. However for us sporting shooters, BC is good enough for most practical purposes.

post-11576-0-40257000-1470575371.jpeg

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Ive used the 0.182 G7 figure that was determined by Brian Litz in the link above, Ive found it to worked quite well out to 550yds in Shooter.

 

I find there is always a little bit of fudging needing to be done between BC and my chrono figures to get Shooter to mirror my real world findings though.

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I've found putting the different BCs into strelok pros multi BC program works very well. Got me within 1/2 moa from 760 to 1644 yards Thursday.

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Us the G7 BC figure for that bullet. Then fine tune or "True" with velocity.

 

Make sure that all your atmospherics are correct when inputting your data.

 

Steve

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Welll,,,,,run some numbers this morning and comparing the manufacturers top figure and a "corrected" applied ballistics BC it does seem that long range shooters would certainly have to take notice but in the real world for ME and my own application I think I can be happy just to plug in the quoted BC and "slightly" adjust from what actually happens on the day,,,,,,,Rarely shoot in excess of 600 in the field and at this distance difference in drop is only 3" and an inch on the wind[5mph},,,,,conditions will mess that up even more,,,don,t we know it,,,,,,When I first saw "Litz,s" figures I really did think I had been missing something fundamental but not quite so,,,,thanks for input above guys,,,,regds,,,,O

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Dave,good points raised.

Simple solution: use G7 values for BC

 

But averaging the given Sierra BCs will be better than just taking the top-or any one-of the three given-so long as the averaging is done sensibly.....see below....

 

 

We all know that velocity is a continuous variable-so long as the bullet is in flight,it loses velocity (but not at the same rate-it isn't 'linear'-and loses velocity faster as flight time (distance) increases- see some data below for bullet velocity at dostances...). Balistic Coefficient is related to speed,so BC also reduces continuously during a bullet's flight. Sierra have pioneeered giving three BC values,one for each of three velocity bands-a simplification,of course,but much better than the single BC typically given,and yes usually the best one,correct only for the highest velocities that cartridge/bulletweight will deliver commercially.

(detailed calculation involves some quite advanced calculus-as variable rates of change are complex,but we don't need it-but realisethat nothing dramatic happens at any one velocty-it's all gradual,and the bands are chosen helpfully,but are arbitrary).

 

OK,as Shuggy suggests,"BC is good enough for sporting use' and indeed at sporting distances,that is fine(I mean about 300 yards)....but which BC?

Brian litz has correctly pointed out (no pun intended) that the common G1 BC is based on a bullet shape that is essentially a cylinder with a rather rounded head (22 rf bullets are like that,flat base bullets with a rounded,rather more aerodynamic head). The G7 profile is much closer to modern (longer range) bullets-boat tail,cylinder,steeper, pointed 'ogive' head. The G7 BC values are therefore much closer to reality and using G7 BCs in the program will help considerably (G7 value is also far less sensitive to velocity,so you don't 'need' several bands-just be grateful that Bryan Litz has a book of them for a wide selection of bullets-see refs below).

 

Big Al reports good results out to at least 550 yards with G7 BCs.Tisme reports that tring the different (sierra band?) BCs helped in LR shooting,and many-eg 247 sniper support the G7 BC-most ballistic programs shoulsd allow it's use.

 

**OK-use the G7 value BC** (if you are using flat base bullets,or short ranges,the G1 BC will be adequate-actually better for the flat base ,and relatively short distance-BC plays a minor role to velocity at 200,even 300y -then BC really matters more and increasingly more (another reason most ballistic solutions are fine for much shooting-when ranges are relatively short,but appear to "wobble" at longer ranges.

 

One reason is that the averaging is potentially complex-your bullet might be travelling more at higher BCs than lower ones (functions of MV and distance).Still,if your mostly in say the top/middle bands ,then averaging those will help (or middle and bottom,if thatiswhere your bullet will be) and that may well be the reason for Dave's report of lowered BCs in some tests.

 

OK..to do all this you need accurate data-as ever-ultimately your rifle/load data,but published data will be reasonably close-there is ultimately no option but to fine tune against your firing data,whatever method is used....

 

Sierra 69 TMK 224 Above 2700 fps best fit BC is .375;2700-1950 .365;1950-1700 .335;below 1700.305

 

Sierra 77 TMK 224 Above 2800 fps best fit is .420;2400-2000 .415;2000-1750 .395;below1700.380has velocities):distance fps:

 

69g@2850 0 2850;100 2580;300 2086;400 1861;500 1653; 600 1466; 800 1171; 1000 1020

 

77g@2750 0 2750;100 2520; 300 2094;400 1898; 500 1714;600 1545;800 1260; 1000 1071

 

 

so,just for example ,shooting those 69s you might average the top BC and next band BC (2700-1956) if your range is out to about 350 yards....because by about that range your bullet will be getting close to1956 fps),if you ar shooting longer,include further band(s) in your average....

 

So,average only the bands you bullet travels through;note it probably won't spend equal time-be affectedby that BC -in each,so a careful weighing (rather than straight average) might help-it's too complex mathematically,but the principle is worth noting- eg if your bullet only travels say 12 yards into a Sierra BC band,best just ignore that last 12 yards.)Just list the relevant Sierra band BC for every 100y (or 50,even 25 yards ) that the bullet is in that band-using the distance velocities & band velocity BCs,and average those BCs (eg at target range 512 yards there will be 5x100y BCs to average (ignoring the last 12 yards:if its 553 yards,use 10x50 yard intervals,average the 10 BCs-ignore the 3 yards....).

 

Always check real firing with the calculations for drop -however it's done-there are just too many (smalll) variabes to get it spot on,but you should be closeenough to fine tune.

 

It may help to have some idea of underlying logic etc,but it is not essential-just do as above.

 

Average with care,if you try it,and for interest-if you have the basic values for BC bands and (your) velocities.Otherwise,the basis for ultimate fine tune that is easiest is the appropriate G7 BC (as per Litz).

 

 

SUMMARY: use G7 BC

 

 

Refs

 

B Litz BergerBullets- "A better Ballistic Coefficient" (why G7 is best,and his book of G7 values)

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Thanks George,,,,Take all that on board,,,,,,,Next time i have to set up a rifle and plot a programme I will try working with an averaging or G7 ,,,,not gonna fiddle with current programmes as they seem to work quite well ,,,,Not much down here to blat at the moment either,,,,cheers ,,,,,D

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:-) the pen-ultimate solution,Dave-move the targets.

 

Ultimate solution,remove them altogether. :-)

 

G

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There is a suite of online ballistics programs at:-

 

http://www.geoffrey-kolbe.com/external_ballistics.htm

 

The drag program is an up-to-date rendering of McCoy's McDrag program which calculated drag from the bullet shape. However, it goes further and uses the calculated drag curve to compute the ballistics for a given bullet. This is done without reference to BCs and standard projectiles.

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I found that the Litz derived G7 works very well with the 69TMK to 600 yards (haven't tried it further). On a nice crisp cold November morning, I adjusted the velocity of my stock 69 load (developed at 65 F) for the prevailing 48 degrees, and used the Litz G7. It got me to within a few inches of the 600 bull first shot on a cold bore. You'll always have to compensate for prevailing conditions anyway, so I'm usually happy if the bullet lands within moa and the Litz derived G7 wasn't far off that.

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Hallelujah !

 

 

Good maths work !!

 

"God may have had to be rather complex in creating the world,but there is no reason to think he was bloody minded." Einstein "Raffiniert ist Herr Gott,aber Borschaft ist er nicht." -a literal translation here,seems clearer than "God does not play dice with the universe" ....a 'crap' translation (IMO :-)

 

gbal

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Ive used the 0.182 G7 figure that was determined by Brian Litz in the link above, Ive found it to worked quite well out to 550yds in Shooter.

 

I find there is always a little bit of fudging needing to be done between BC and my chrono figures to get Shooter to mirror my real world findings though.

Plus 1 on this . I've used StrelokPro,shooter and various other programs. The truing function on applied ballistics is a godsend. Whereas before I had to mess about with mv or bc to get my results to match up with field conditions. On applied ballistics at certain yardages I have corrected the actual field conditions to correlate with the program. This has my program matching to within an inch out to 800 yards.

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