Ballistic Coefficient

English Mike · Posted: Sat Sep 02, 2017 5:18 pm Post subject: Ballistic Coefficient

I note that several manufacturers are now giving two differing figures for a bullet's BC: G1 & G7.
Berger is one & they have an explanation here that makes a lot of sense.

My question is: Will this affect the accuracy of trajectory projections when using PointBlank?

Elvis · Posted: Sat Sep 02, 2017 5:42 pm Post subject: Re: Ballistic Coefficient

there is also different figures for different speeds......you can end up chasing your tail if shooting really long as figure will change as speed drops.

aim and shoot . dont try for range of more than say 350 yards and its all just gobbledegook anyway LOL.

slimjim · Posted: Sat Sep 02, 2017 6:54 pm Post subject: Re: Ballistic Coefficient

English Mike, what bullet are you trying to use? I've done some BC testing to check between actual and advertising.

www.huntingnut.com/ind...=ballistic

For those not familiar with G7, it is better optimized for modern low-drag bullets. G1 and G7 are distinctly different values and require different algorithms to generate trajectories.

English Mike · Posted: Sun Sep 03, 2017 4:54 pm Post subject: Re: Ballistic Coefficient

slimjim wrote:

English Mike, what bullet are you trying to use? I've done some BC testing to check between actual and advertising.

www.huntingnut.com/ind...=ballistic

For those not familiar with G7, it is better optimized for modern low-drag bullets. G1 and G7 are distinctly different values and require different algorithms to generate trajectories.

Moly coated .243 Lapua Scenar 90gr which definitely resemble the G7 profile (boat tail, long ogive, open tip).

Using the G7 profile gives a lower BC than the G1, which confuses the heck out of me.

ETA I just read through all the posts in your link & that helps a great deal.
I have at least 10 different varieties of .277 bullets sitting in Albuquerque if you ever want any more to play with, though I can't tell you what exactly for another eight weeks, which is how long it'll be till I'm out there again.

slimjim · Posted: Sun Sep 03, 2017 7:05 pm Post subject: Re: Ballistic Coefficient

English Mike wrote:

Using the G7 profile gives a lower BC than the G1, which confuses the heck out of me.

The G7 BC can't be used in typical ballistic calculators designed for G1 inputs. I don't know if PointBlank handles G7 BCs? If it doesn't I would use the calculator on the Berger or Applied Ballistics websites. They are compatible with G7 inputs.

Vince · Posted: Sun Sep 03, 2017 8:13 pm Post subject: Re: Ballistic Coefficient

Mike, unless you are looking for Bench Rest accuracy I tend to agree with Elvis. Pick a projectile that you think will meet your needs and try it out. If it doesn't perform as you hoped, or you can't find a load that works, then look at a different projectile.

I love what Slim does with his testing, but it goes beyond anything I will ever need or do, however it 'scratches an itch' that Slim needs to scratch, and that's great. I think that with the multitude of different projectiles on the market today, the average shooter/hunter gets confused and too wrapped around the axles with advertised claims. Said shooter then gets discouraged when they don't always achieve those claims.

I look at my target species, look at a type (HP, SP etc) and weight of projectile to humanely harvest that species then develop a load that will give me minute of critter. Anything beyond that, I find, creates confusion leading to not trusting your loads.

Elvis · Posted: Sun Sep 03, 2017 10:56 pm Post subject: Re: Ballistic Coefficient

very sage words there Vince....very sage indeed, Scratch

are you sure you not related to Gelandagans lot way back somewhere???? Very Happy

DallanC · Site Admin Joined: Jan 18, 2005 Posts: 3572 Location: Utah

PointBlank is based on a G1 table. I've rarely had people ask for G7 based computation, but not enough to justify the work to implement it.

Plus.... Sierra uses THREE different BC's for their bullets, depending on the velocity range you shoot them in. Its all pretty technical, all a Ballistics Calculator helps with is giving a representation for a theoretical trajectory.

-DallanC

Vince · Posted: Tue Sep 05, 2017 3:56 pm Post subject: Re: Ballistic Coefficient

Elvis wrote:

very sage words there Vince....very sage indeed, Scratch

are you sure you not related to Gelandagans lot way back somewhere???? Very Happy

anything is possible in this day and age mate.

I see Dallan has answered your query, so sorry if I have hijacked your thread he of Pomgolia.

slimjim · Posted: Wed Sep 06, 2017 6:01 pm Post subject: Re: Ballistic Coefficient

DallanC wrote:

Sierra uses THREE different BC's for their bullets, depending on the velocity range you shoot them in.

During my BC testing I found several bullets whose BC (G1) noticeably changed when tested at different speeds. It appropriate for the bullet, a G7 BC provides better drag/trajectory calculations over the more significant velocity change associated with long range shooting.

DallanC · Site Admin Joined: Jan 18, 2005 Posts: 3572 Location: Utah

Yea, the big problem is most mfgs dont give out a G7 based SD for their bullets. Everyone is too used to G1 values.

-DallanC

English Mike · Posted: Sun Sep 10, 2017 3:27 pm Post subject: Re: Ballistic Coefficient

I know real world shooting is ultimately the only way to know exactly how a bullet behaves from a particular firearm & I guess that's really the point: I'm looking for excuses to shoot more. Very Happy