Ballistic coefficient estimator

[NSP64]

How accurate is this estimator. Its a free one on the web, I think the designer is a member here. I can only shoot 100 yrds and was wondering if anyone else uses it and how well it works. I was asking because Lee lists its 170 gr RNFP .308 mold as having a BC of .268 and the BC estimator shows
.454 ?

[Larry Gibson]

Obviously that BC is not correct. Even jacketed 170 gr FPs don't have BCs that high. I've measured the BCs (M43 Oehler with screens at muzzle and at 100 yards) of Lyman's 311041 and it runs .220 - .200 at 1800 to 2200 fps.

Larry Gbson

[NSP64]

Now would the BC go up as velocity falls, then start to drop off? Seems like I remember bullet mfg listing BC's based on velocity.

[Larry Gibson]

Now would the BC go up as velocity falls, then start to drop off? Seems like I remember bullet mfg listing BC's based on velocity.

Some do, some don't....all depends on the bullet shape and velocity range.

Larry Gibson

[williamwaco]

The Lee 170 RNFP is very similar to the Lyman 311041.

The Lyman Cast Bullet Handbook No 3 ( Brown Book ) lists the BC of the 311041 at .220.

Way to go! Larry


.

[Tom Myers]

How accurate is this estimator. Its a free one on the web, I think the designer is a member here. I can only shoot 100 yrds and was wondering if anyone else uses it and how well it works. I was asking because Lee lists its 170 gr RNFP .308 mold as having a BC of .268 and the BC estimator shows
.454 ?

You might be referring to the BC estimator that is posted on the TMT Enterprises Precision Ballistics Website at this link.

Precision B.C. Estimator (http://www.tmtpages.com/calcbc/calcbc.htm)

If so, the dimensions may not be entered correctly.

If you are indeed referring to this site, the most common error in entering data is that of entering the entire nose length instead of the just the ogive length. Reading the Cast Bullet diagrams in the site instructions will show a Representative image of the RCBS bullet which is similar to the Lee Bullet and details the dimensions to be entered for that bullet. Substituting the measurements of the Lee bullet in the calculator should return a reasonable BC for the Lee bullet

http://www.tmtpages.com/calcbc/rcbs40.gif



I do not have the actual dimensions of the Lee C309-170-F bullet but, I scaled out the dimensions of the image on Lee's website and entered them into the Precision Cast Bullet Design software (http://www.tmtpages.com/draw/draw.htm) to verify the integrity if the values and it appears to be quite close..

http://www.tmtpages.com/LinkSkyImages/forum_images/Lee_Mold_C309-170-F.gif

http://www.tmtpages.com/LinkSkyImages/forum_images/Lee_Mold_C309-170-F_170_gr_Sketch.Jpg

The website calculator uses a slightly different algorithm to calculate BCs than does the bullet design software but entering the dimensions into the website BC Calculator returned this result which is quite close to the design BC.

http://www.tmtpages.com/LinkSkyImages/forum_images/WebBCEstimator.gif