AK Rifles banner
1 - 10 of 10 Posts

·
Registered
Joined
·
1,648 Posts
Discussion Starter · #1 ·
almost everyone knows of the term "Ballistic Coeffecient" (B.C.) but who really understands what it means ? the 'ole G1 B.C. # that you've all come to a misunderstanding with is wrong in more ways than one . this method of MANIPULATING G1 to fit every bullet was developed by Sierra as a SALES GIMIC . they also needed something that worked better with THEIR bullet shapes as the military developed 'G' models wernt a good match with them . HOWEVER , with our milsurp weapons and ammo , G1 becomes an bassacwards joke . the military BC models are just right for us !

used properly , the MANIPULATED G1 MODEL " will do for a quicky" but , many don't use it properly or even understand how or why .

Here is Sierras ballistics page . Some is good work and informative , some is sales gimic and propaganda . Reader discression advised .


http://www.exteriorballistics.com/ebexplained/index.cfm


there is a much better place to start from here :


http://www.steyrscout.org/extbal.htm


but , skip them till later . here is the bullet G1 is based on :



obviously , this bullet can not model the military bullets we use . its not even close .

here is a real life example .


recently i discoverd a DOPPLER RADAR graph for TCW 762x39 WOLF hollow point boat tail ammo in a criminalistics journal , article by Lucien C. Haag . please note the difference in shape between the KRUPP/G1 above and previous pics of WOLF 762x39 HPBTs and FMJBTs posted in this forum .

TABLE 2

7.62x39 122 gr. JHP-BT (Tula mfg. )

ABBREVIATED EXTERIOR BALLISTIC RESULTS

from

YPG DOPPLER RADAR TRACK #101 12/5/98

muzzle elevation : +5.00 deg





DISTANCE _____VEL. _______________FLIGHT TIME __CALC. G1 B.C.

m ____________m/s______ (f/s)________ sec

muzzle ________710.6 ____(2331)______ 0.0000 -

100 __________ 606.9____ (1991) ______0.1530________ 0.257

200 __________ 520.5 ____(1708) ______0.3318 ________0.281

300 __________ 444.0 ____(1457) ______0.5403 ________0.280

400 __________ 378.0 ____(1240) ______0.7830 ________0.264

500 __________ 330.3 ____(1084) ______1.068 _________0.258

1000 _________ 242.7 _____(796) ______2.858 _________0.305





___________________________________overall avg. B.C. = 0.285

Gun position 199 ft. MSL, temperature 60F, Relative Humidity 40%, barometric pressue 998.5 mbs

also notice that to get a G1 # to work you have to RE-CALCULATE the bullets B.C.# EVERY 100M or so . most ballistic programs dont have a way to do this . Sierra generally provides several G1 B.C.#s for certin velocity zones with every bullet to cover normal hunting ranges and more for target applications but , others do not . Even when done correctly this MANIPULATION can not PREDICT the true flight of any bullet whos shape greatly deviates from the G1 model .

several years ago i measured the same early TCW/WOLF made HPBT bullet on JBM ballistics website and got 0.178 G5 at nearly the same muzzel velocity as above . G5 being the closest SHAPE MATCH .





when i plug THIS CORRECT G5 # into the conditions reported for the doppler radar shot , i come up with a trajectory virtually identical except that at 100M i'm only +18fps fast , at 400 and 500M each , i'm 9 fps slow and then +4fps at 1000M . the reason for these very slight velocity discrepancy ? Imperfect compensation for discrepancy in shape compared to the G5 model , especially in the important nose and base profiles . of course , there are other considerations but they are negligable given NOMINAL flight .

Campare my Wolf HP bullet to the G5 MODEL above ;



total length = 3.316 calibres

nose length= 1.871 calibres (truncated meplat)

tangent radius = 6 calibres

boat tail length = 0.483 calibres

base diameter = 0.871 calibres



once you find the the CORRECT B.C. MODEL for that bullet no further calculations are nessisary save plucking out the B.C. # for (X) muzzel velocity . its trajectory closely follows the MODEL .

then there is the G6 model .




this is the spittin image of the infamous 303 British MK7 bullet .



it works very well for M67 type , 762x39 FLATBASE bullets with ~6s calibre radius nose profiles and is a much better match to the still shorter ~5s to ~5.5s radius bullets than G1s' 2s radius .

i dont think you'll find any bullets similar too INGALLS and G1 outside the realm of paperpatched smokepoles .

all this mite seem like trivial hair splitting and really , it is for most applications . that is , untill you try to back-engineer AK/SKS rear sights and military doctrine or discover that certin bullets loose far more velocity/energy at range than the makers ballistic tables show , or , use to .


the correct BCs' for 762x39 bullets @2340fps muzzle velocity


WW ,REM ,FED 762x39 FMJ and SP FLAT BASE bullets . The Sps tend toward the low number , FMJs to the high number . These bullets are all 0.840" to 0.850" long with a ~5s tanget radius ogive . i call these US67 bullets
= .140 to .145 G6

IN BETWEENS 762x39 bullets . barnaul SPs and HPs , Uly 8M3 HP , early rem FMJ . These are all 0.860" to 0.890" long
= .145 to .150 G6

Sierra 125gr .311" SP / PMC Softpoint , 0.880" long . True 6s calibre tangent radius ogive .
= .160 G6

6 x .311" = 1.866" = 6 s( s = segment of a circle) calibre (.311") tangent (besides or next too) radius ( ? dia.of a circle) ogive (the curved forward part of a bullet) .







MILITARY M67 FMJ bullets , 0.905" to 0.925" long
= .164 G6 works for all

An idealized YUGO M67 bullet , 0.941" long would be
= .167 G6

this is it in millimeters



HPBT BULLETS . 0.990" to 1.035" long
Wolf COPPER HPBT
= .172 G5

early HPBT with plastic base insert in Klimovsk and Ulyanovsk 762x39 ammo
= .175 G5

Wolf HPBTs (normal bi-metal)
= .176 to .180 G5 depending on production lot

FMJBT bullets (M43 shape and weight)
= .192 to .196 G5 depending on maker and production lot
= .156 G7 (G5 is a closer match but G7 is worlds better than any G1 # without multipul averages)

estimate of HORNADY V-MAX from pics . ~10s calibre secant nose profile .
= .218 G5


154gr Wolf SPBT
= .205 G5 @2100fps muzzel velocity
 

·
Registered
Joined
·
1,360 Posts
"...untill you try to back-engineer AK/SKS rear sights and military doctrine or discover that certin bullets loose far more velocity/energy at range than the makers ballistic tables show , or , use to . "
Ucertainties about actual impact points at 300+ meters? My Romanian rear sight gives a 12-13 inch difference between the 100 and 300 settings. A six o'clock hold is mandatory and, once you get used to it, is not bad since you see all of your target at 300 meters/yards. Could this be intentional, to make sure the point of impact is dead on or high but never low?
 

·
Registered
Joined
·
1,648 Posts
Discussion Starter · #3 ·
sorry for the delay :sad: still haveing puter trouble . scrolling or moveing causes blurring . 20 other puter problems too :evil:

are you sure the distance was 100M for the 12"/13" diff between 1 and 3 ?

was that with the exact same ammo from the same box/lot ?

did you do a 'group center' find or just guestimate it ? i just use a 3shot group and find center by joining them with a marker and crossing over from point to the middle of the opposite side .

did you shoot groups rested from a bench or bags the exact same way both times ?

how easy was your target to see/line up on ? this can be a big factor .

did the light change between the -1- group and the -3- group ?



just trying some trouble questions on ya .


the rearsite settings should hit close to

-1- , 00
-2- , +4"
-3- and battle symbol , +10"
-4- , +17.5"
-5- , +27"
 

·
Registered
Joined
·
1,360 Posts
I'm a poor one for consistancy. I shoot from Buddha sitting, Russian style rectangle target with aiming point center of bottom edge, 100 yards, and use the Soviet method to find the center of a four shot group. If I zero elevation to strike Point of Aim at 100 yards with the sight set to 100 I always have to float the target at 300 yards with FMJ. HP's hit lower. Not a problem, just interesting.
I let a doctor work on my eyes with a sharp knife so now I'll have to do it all over again.
Good ballistic coefficient stuff.
 

·
Registered
Joined
·
1,648 Posts
Discussion Starter · #5 ·
5thShock said:
If I zero elevation to strike Point of Aim at 100 yards with the sight set to 100 I always have to float the target at 300 yards with FMJ. HP's hit lower. Not a problem, just interesting.
its the diff in powder burn / portpressure and average velocity btween the 2 loads , not BC

the diff between FMJBT and HPBT is like 1.5" @300m , all else being equal .

5thShock said:
I let a doctor work on my eyes with a sharp knife so now I'll have to do it all over again.
chemo does wonders for vision also . seems im gettin off easy .

5thShock said:
Good ballistic coefficient stuff.
thanks .

it dont matter much to just 300M trajectory wise for the average joe , shootin his AK . hope it was a good vehicle for helping others grasp an understanding of what B.C. is/means .
 

·
Registered
Joined
·
1,648 Posts
Discussion Starter · #6 ·
when calculating a 7.62mm bullets BC , one of the seemingly difficult tasks is to find the bullets true nose length and then the ogive radius . its really rather simple .

bullets for the 762x39 normally have a nose length (what counts as 'ogive radius') of .500" too .610" , here's a quick and dirty way to get a real darn close measurement . pull a bullet from a loaded round and re- insert it backwards ;



hey look ! nearly all the 'ogive' of this one went into the case . the bullets bearing surface wont without force because cartridge brass springs back around .0005" of an inch , give or take .

measure the case length :



just round it off too 1.505" is fine . then measure the case + inverted bullet useing no more pressure than the calipers friction mechanizm allows ;



again , just rounding it to 1.810" is ok . so , 1.810" minus 1.505" = .305"

ok , now measure the bullet .



.905" minus .305" = .600"

the nose length is .600" and it just so happens that with 7.62 calibre the ogive radius will be about 6 calibres .

here is a Chinese steel core 'PS' bullet . as you can see , slightly more bullet 'ogive' ahead of the crimp enters the case ;



these are generally considered to be 6.1 calibre radius based on a .308" bullet with the bearing surface bumped up , rather than on a .310" or .311" bearing . why ? who knows ?

here is the other end of the spectrum ;



that's a lot of bearing surface 'outside' the case huh ? MOST US made bullets loaded by the big 3 are like this , a .500" nose length / 5 calibre radius ogive . this makes a real difference in BC. .

here is the Lapua S.405 bullet ;



only a little extra outside the case , ahead of the crimp . this one has about .570" nose length .

theoretically , all M67 and M43 bullets should have .610" nose length

maybe none of this matters for the average joe shooting an SKS or AK47 out to 300M but it does matter shooting farther like , for an RPK gunner and shooting 'stuff' at longer ranges .
 

·
Registered
Joined
·
212 Posts
Yoda,
Been reading this all with a lot of interest. Kinda curious though, does the equation for the calculation of B.C. require the use of calculus? Was just wondering if it is requires differentiation or integration or if it is a differential equation that you are using to determine B.C. ?
I was hoping that maybe if it is an equation or for that matter an generated algorithmic value would you be so kind as to put it into print here?
Wouldn't mind takin' a crack at an equation as I think that I am properly prepared to do so ( got my calipers and scientific calculator ready to go and my head is wrapped in aluminum foil to help focus my thoughts... :grin: )
This has been the most interesting thread I've read in long while, very, very cool stuff especially since I am a calculus nut.
Thanks again for all the cool info in all your threads !
 

·
Registered
Joined
·
1,648 Posts
Discussion Starter · #8 ·
let the 'puters do the trig and calc :wink: but you can find all them formuli here ;

http://www.eskimo.com/~jbm/

along with several methods of finding B.C.

a really cool fellow too i mite add . i've communicated with him several times about his website .

enjoy
 

·
Registered
Joined
·
1,360 Posts
The first electronic computers were created in England and here to do just this kind of thing for cannons. The calculations were many and heavy.
 
1 - 10 of 10 Posts
Top