G1 or G7 how can I tell

[donb31]

I am loading sierra Match 69gr. bthp bullets to shoot a 500 yard match. There ballistic calculator says the ballistic Coefficient is.301 >2800 ft/sec so how do I know if this is a G1 or G7 number it seems to matter a great deal. Thanks everyone for the help.

 

[Laurie]

It's G1. The G7 BC for this bullet is 0.169. (Bryan Litz Applied Ballistics for Long Range Shooting 2nd ed.)

There is no .224" bullet available on the market with a G7 BC as high as 0.3. The Highest that Bryan L has tested is the 90gn Berger VLD with 0.281 G7 / 0.551 G1

 

[r bose]

The 69g sierra is a great bullet but at that distance, you might be in for getting pushed around a lot. Maybe it's what you have, your twist, or what you can find but if possible, you might want to check out something heavier, better BC

 

[Bulletman]

All BC 's from Sierra are G1

 

[JamesnTN]

The G1 is based off of a FB round nose projectile such as say round nose pistol bullets where the G7 model is based on the VLD profile bullets and when setting up your ballistics you should use the G7 profiles.

 

[CatShooter]

The G1 is based off of a FB round nose projectile such as say round nose pistol bullets where the G7 model is based on the VLD profile bullets and when setting up your ballistics you should use the G7 profiles.

Not really...

The G Model

In 1881, the Commission d'Experience de Gâvre did a comprehensive survey of data available from their tests as well as other countries. After adopting a standard atmospheric condition for the drag data the Gavre drag function was adopted. This drag function was known as the Gavre function and the standard projectile adopted was the Type 1 projectile. Thereafter, the Type 1 standard projectile was renamed by Ballistics Section of Aberdeen Proving Grounds in Maryland, USA as G1 after the Commission d'Experience de Gâvre. For practical purposes the subscript 1 in G1 is generally written in normal font size as G1.

The general form for the calculations of trajectory adopted for the G model is the Siacci method. The standard model projectile is a fictitious projectile used as mathematical basis for the calculation of actual projectile's trajectory when an initial velocity is known. The G1 model projectile adopted is in dimensionless measures of 2 caliber radius ogival-head and 3.28 caliber in length. By calculation this leaves the body length 1.96 caliber and head, 1.32 caliber long.

Over the years there has been some confusion as to adopted size, weight and radius ogival-head of the G1 standard projectile. This misconception may be explained by Colonel Ingalls in the 1886 publication, Exterior Ballistics in the Plan Fire; page 15, “In the following tables the first and second columns give the velocities and corresponding resistance, in pounds, to an elongated one inch in diameter and having an ogival head of one and a half calibers. They were deduced from Bashforth's experiments by Professor A. G. Greenhill, and are taken from his papers published in the Proceedings of the Royal Artillery Institution, No 2, Vol. XIII.” Further it is discussed that said projectile's weight was one pound.

 

[Syncrowave]

The G1 is based off of a FB round nose projectile such as say round nose pistol bullets

Not quite. More like this:

All BC 's from Sierra are G1

Yep. I called them once to ask, and that's what they told me.

 

[Laurie]

As CatShooter says, it's a concept as are all 'reference projectiles' used as a basis for comparison to obtain BCs. However, if you want to imagine what a G1 reference projectile looks like, consider a typical run of the mill 0.22 Long Rifle 40gn LRN projectile, heeled base aside.

In an exchange I had with Bryan Litz some years back about .22 rimfire ammo ballistics, Bryan not only confirmed that G1 BC based ballistic program calculations were appropriate for these bullets, but as an aside threw in that the standard form is very close to that of the theoretical 'G1 reference'