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fatelvis
12-15-2007, 04:55 PM
I'm trying to punch in the necessary data in Sierra's ballistic program, and hit a snag. How could I find the BC for my Lyman #358627 cast bullets? They no longer make the mould, and do not have it listed anywhere I have looked. It is .358" diameter, has 2 crimp grooves, and weighs 218 grns lubed and checked. Any help would be appreciated.

freedom475
12-15-2007, 05:15 PM
BC is really a "crap shoot" even with your ballistic computer. There are so many variables that can change BC on a day to day bases.

I have tried to match loads to my "fancy" scopes like the Ballistic Plex, Shepard, my 50BMG Swarovski Barret optic, and BPCR sights, I have found that the only way to tell where the bullet will hit that perticular day is to shoot them at paper.

The ballistic computer will get you close but there not the Holly Grail when it comes to actual boolit impact. Even the Gov. uses big screens at graduated yardages to find BC.

I know this doesn't really answer your question, but the best way is to get out there and shoot.:drinks::Fire:

Scrounger
12-15-2007, 05:29 PM
Chronograph the same bullet/load at two different ranges, say 10 yards and 50 yards. The velocity loss is determined by BC and therefore can be used to find the BC. There should be a formula somewhere on the Internet for computing the BC using the two different velocities. Ideally you should have two chronographs and get your velocities from the same shot; using one chronograph and moving it and using two shots will introduce some error into the data, but considering it is a pistol bullet/round, conceived for close4 range shooting, it should be adequate.

http://en.wikipedia.org/wiki/External_ballistics

Freeware small arms external ballistics software

accurateshooter.com Ballistics section (http://accurateshooter.com/) links to / hosts these 4 freeware external ballistics computer programs:

[1] (http://www.cronander.net/CRONXR1A.zip) 2DOF & 3DOF R.L. McCoy / Gavre exterior ballistics (zip file) - Supports the G1, G2, G5, G6, G7, G8, GS, GL, GI, GB and RA4 drag models
[2] (http://www.huntingnut.com/files/pointblank/PointBlankCRBSv18a.zip) PointBlank Ballistics (zip file) - Siacci/Mayevski G1 drag model
[3] (http://www.eskimo.com/%7Ejbm/ballistics/traj_basic/traj_basic.html) JBM's real-time interactive online ballistics calculator
[4] (http://accurateshooter.net/Downloads/pejsajacksonballistics.xls) Pejsa Ballistics (MS Excel spreadsheet) - Pejsa model
[5] (http://www.bestpalmsoftware.com/mathematic/misc/pocketssf_sharpshooterfriend344.htm) Sharpshooter Friend (Palm PDA software) - Pejsa model
[edit (http://en.wikipedia.org/w/index.php?title=External_ballistics&action=edit&section=19)] See also

BAGTIC
12-15-2007, 05:37 PM
One problem is that B.C. varies with bullet velocity. Most bullets will typically have at least three B.C.'s for different speed ranges.

BCB
12-15-2007, 06:44 PM
Scrounger is correct about the 2 different velocities at 2 different ranges. Use these number to calculate BC. There is an equation, and I haven't used it for a good many years, but it did produce fairly repeatable and predictable performance. i.e. I could use it to predict impact at, say, 125 yards with a 44 Magnum load when it was zeroed at some known range. Only problem--you best have a load that is shooting accurate as those Chronys get pretty small at ranges greater than 25 yards!!!!!!!!!!!!!!! Good-luck...BCB

Tom Myers
12-16-2007, 12:20 AM
Try this. It will give you fairly close estimation of the BC for a cast bullet.

http://www.uslink.net/~tom1/calcbc/calcbc.htm

Tom Myers
Precision Ballistics and Records (http://www.tmtpages.com)


I'm trying to punch in the necessary data in Sierra's ballistic program, and hit a snag. How could I find the BC for my Lyman #358627 cast bullets? They no longer make the mould, and do not have it listed anywhere I have looked. It is .358" diameter, has 2 crimp grooves, and weighs 218 grns lubed and checked. Any help would be appreciated.

Tom Myers
12-16-2007, 02:45 PM
I had some play time this morning so elaborating on the BC of the Lyman 358627 seemed like a fun thing to do.

I found a sketch of the bullet in question in a Lyman Cast Bullet Reference and scaled up a drawing on the Cast Bullet Design and Evaluation software that I am developing.

http://www.tmtpages.com/LinkSkyImages/Misc/Lyman-358627.gifhttp://www.tmtpages.com/LinkSkyImages/Misc/Lyman_Mold_358627_209_gr.gif

This software estimates the ballistic coefficient of the bullet being designed or evaluated so I thought that I would compare the estimated BC with my On-line Ballistic Coefficient Estimator (http://www.tmtpages.com/calcbc/calcbc.htm).

Using the values from the dimension sketch of the Bullet Design software

http://www.tmtpages.com/LinkSkyImages/Misc/Lyman_Mold_358627_209_gr_Sketch.gif

I plugged them into the On-line Ballistic Coefficient Calculator and arrived at a BC of 0.260 which is higher than the 0.2473 of the Designer software. However the Designer software takes into account the meplate radius, whereas the On-line estimator is not quite that sophisticated.

http://www.tmtpages.com/LinkSkyImages/Misc/Estimator.gif

Anyway, it is as previous posters have stated, estimating or calculating the ballistics coefficient for a particular bullet is closer to a crap shoot than a science as variables such as atmospheric conditions, muzzle velocity, bullet concentricity, bullet stability and bullet deformation can, and do, cause major variations in the average BC of a bullet throughout the trajectory.

A Ballistic Coefficient calculator, or estimator can be effectively utilized to obtain a preliminary trajectory that may then be compared to an acutal trajectory and the BC may then be adjusted to conform to the actual trajectory.

An accurate BC can be calculated through measurements of velocity differences between carefully measured ranges and also, if the muzzle velocity is known, from precise measurements of sight adjustments and group centers on targets at carefully measured ranges.

Isn't this fun?

Tom Myers
Precision Ballistics and Records (http://www.tmtpages.com)

fatelvis
12-16-2007, 02:50 PM
Thanks a million guys! I'm going to average the two, and consider it .253 BC.

felix
12-16-2007, 03:00 PM
Tom, you do excellent work! ... felix

Blammer
12-16-2007, 03:35 PM
Heck just put .500 on it an run with it!


:)

pa_guns
12-16-2007, 03:42 PM
Hi

A crude way to check things out:

Shoot the Lyman through the chrono and see where it hits.

Do the same thing with another bullet of roughly .25 BC at the same velocity. See where it hits.

You need to do both through the same rifle, same target, same conditions. Once you correct for the inevitable velocity difference you can judge where the two bullets are relative to each other.

Bob