B.C./Drop Report--MUST READ&COMMENT

[MontanaRifleman]

I was thinking about this and the question came to mind. Is figuring BC based on trajectory only taking into account the BC from the point where the bullet and scope are zeroed out? Should I be trying to look at it as total drop from the muzzle to range b and c to find a BC that isn't infuenced by scope hight or zero range and height?.

BC isn't influenced by scope hieght meaning that changing scope doesn't change BC. But changing scope height can make 2 or 3 set's of groups more closely fit a particular trajrctory or a range of trajectories.

The more data points you look at from muzzle to down range the better your conclusion will likely be. And the more times you repeat it, the more conclusive it will be. Repeatability is key to making reliable conclusions.

 

[LRSickle]

I shoot the same setup, a 257Wby, 100gr tsx at 3725fps. I figured via the two Crony method that my the bc of my 100gr TSXs was .426. I'm not sure what the published bc is but this bc has worked really well in the two programs I use.

 

[sp6x6]

Im sorta new to this but a mv 3500 fps= to a velosity around 3284 fps and at 100 yds. At 300 yds the velosity is 2892 give or take. the drop at 300 yds is around 15 in, and 27 in at 400 yds. The bc may also be incorect. Acording to the math it is .42. I don't know but my math may be all wrong.gun)

7 Loader, tOO help you out look at the beginnig of thread, he basically has a 300 yrd zero, which relates to other drop charts listed

 

[RockZ]

MR,
What drops do you get when you run a .8-.85 BC?
Just curious what your program gives.

 

[MontanaRifleman]

RockZ,

Zeroed @ 320yds, if I use 3500 fps, 5000' elev, and a sight height of 2.2" with a BC of .4 I get

100 - +2.3, 300 - +1.0, 400 - -5.9

With a BC of .8 I get...

100 - +1.9, 300 - +.08, 400 - -4.8

Change sight height to 1.8" (which my scope is with low rings) and BC of .4 I get...

100 - 2.6, 300 - +1.0, 400 - -6.0

With a BC of .8

100 - 2.2, 300 - 0.9, 400 - -4.9


Changing the sight has a fair amount of affect on close range trajectory, but overall, not real big changes in trajectroy with HUGE changes in BC at those ranges and velocity. And IMO, 3500 fps is a little on the slow side for 100 gr bullet out of a 257 Wthby Mag, but maybe that is the real velocity. A change of 50 fps changes the trajectories by 0.1"

Here are a couple of online calcs that I use

External Ballistics Calculator

JBM - Calculations - Trajectory

 

[RockZ]

MR
Thanks for the data.
You are right they are close due to the short range but it looks like .8 fits it more closely.

 

[eddybo]

Dunno , but if you plug the info into exbals trajectory validation option is gives you the old "calculations do not converge" error. I do not think your going to figure very much out about the BC of a bullet only shooting to 400 yards anyway. I am going to have to go with the ......not enough information to answer answer option.
That said I would sure like to have a .8 BC 100gr TSX that I could push 3500fps with accuracy.

When I hear stuff like this from experienced shooters, the only thing that comes to mind is error stacking.

 

[RockZ]

It shows the tenuous nature of drops/bc's and ballistics in general. You need very precise data to come to genuine conclusions.
The report I was quoting is from the November 2009 issue of GUNS magazine from an article named
"Ballistic Coefficient"
Here's a link -go to page 16
http://www.gunsmagazine.com/digital/GN1109.htm?page=16