How to determine B.C.

DANTEC

Well-Known Member
Joined
Jul 6, 2002
Messages
390
Location
France
most commun way is to ge the BC of an unknow bullet is :

get velocity at the muzzle and get the velocity at a know range (for example 300 yards ) by using two chrono in the same time

after you have your two velocity you run the values in a software and the software give you the BC

I use QUICKLOAD software to do this job

Good shooting

DAN TEC
 

grkldoc

Member
Joined
Feb 5, 2002
Messages
9
Location
Danville, PA
You have to have two very accurate chronographs. I Did this with two Beta chronies ended up with a range of values from .687 to 1.3 for the LRB 120gr J36. This was only at 100 yds. though. Perhaps at 300 yds., there would be enough difference in the velocities to downplay the intrinsic error in the chronies.

[ 08-24-2002: Message edited by: grkldoc ]
 

DANTEC

Well-Known Member
Joined
Jul 6, 2002
Messages
390
Location
France
dear GRKLDOC

check your Chrony is easy with home stuff

put your two Chrony one just on the rear of the other at 3 feets and shoot a Eley TENEX or any match ammo in 22 RIMFIRE match from a good quality rifle ( as ANS54 ) at read the value on

1 bullet shot
average of 5 shots
average of 10 shots

by this way you can check if the reading is near the ammo maker value , 22 rimfire are pretty good in velocity calibration .

for hight velocity AR15 in 223 with military ammo is good too .

for velocity testing you need to get enought velocity drop to avoid calculus error
best is 10 meters and 300 meters that give you enought velocity drop .

shot 5 shots and make average on velocity by this way you reduce reading error of your CHRONY .
Note : I know that J36 are costly but you can make atest with any bullets .

good shooting

DAN TEC
 

CatShooter

Well-Known Member
Joined
May 8, 2001
Messages
1,224
Location
South Windham, CT - in Moo-Cow country
grkldoc...

Check both chronys to make sure that they are dead on... set them both up at 20 or so feet, and do as Dantec said... shoot a consistant match grade .22 rf through them... 20 rounds each to get a accurate average, before you go any further.

Also, remember that the close chrono is five to ten feet in front of the muzzle, you need to have 100 yds (or better 200 to 300 yds) between the center of the chronos, not from the muzzle, for an accurate velocity lose calculation.

Depending on your software, you might have to measure the velocity at several distances, because the software only recognizes G1 form factors (like Sierra "Infinity"), and these are not accurate with just two readings (especially with 100 yds separations).

If you are shooting the LRBT bullets, how are you finding their accuracy... they are touted as the most accurate bullet in the world, but I have yet to find anyone that has had that kind of accuracy.

If you have (or can borrow) the 4th eddition of the Sierra loading manuals, there is a lengthy discourse on why their BS have changed values (mostly DOWN) over the years. They explain the problems they had with inacurate instruments, errors in trap spacing, etc... it is not the easiest thing to do with accuracy (as you have already seen), as there are lots of gremlins waiting to bite you in the butt.

CatShooter.

[ 08-24-2002: Message edited by: CatShooter ]
 

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