The difference between G1 and G7

[caerhays sniper]

Hi chaps just wanting to know the difference between these 2 on strelok pro and which i should be using? Cheers Jim

[farmer7]

I tend to use G1 for flat base bullets and G7 for boat tails. Probably an over simplified answer but it works for my needs!

[1967spud]

farmers ansa is spot on for simplest terms the two different g numbers are based on different profile bullet cross sections form facters g1=fb g7=bt

[Laurie]

Both models compare the measured or calculated drag curve of a bullet design against that of a standard 'reference bullet'. The G1 type wasn't even a bullet but something akin to the turn of the 19th / 20th century 1-pounder 'Pom-Pom' shell, a short very blunt round-nose flat-base type. The G7 reference is also an artillery shell, but a modern long-nose, long-boat-tail type whose shape is very close to that of a high performance boat-tail match bullet.

 

The comparison gives a drag-based value called the 'form-factor' expressed as the lower-case letter i followed by the model, ie i1 or i7 here. It is a numerical appreciation of the projectile's efficiciency as a comparison to the reference model. So a bullet with an i1 or 17 of 1.0 is equally efficient as the original G1 or G7 projectile. As it's drag based, a higher figure is 'bad', a lower figure is 'good'. The most efficient 6.5 and 7mm long-range match bullets have i7 values close to, even occasionally below values of 0.9, while the long-established .30 168gn Sierra MK runs at 1.161 or is 16% less efficient than the G7 reference. There are BIG differences betweern i7 values of various match and long-range sporting bullets, especially in .30 calibre.

 

The other aspect of bullet ballistic performance is its sectional density (SD) which is a calibre to mass ratio value, and which applies to G1, G7 or wehatever and any and every 140gn 6.5mm bullet has an SD of 0.287, and every 155gn .308 bullet is 0.233 and so on and it doesn't matter which reference bullet is in use. However, combine SD and i-form value and you get the BC, Ballistic coefficient, the type depending on the reference projectile. Therre are many other reference projectiles, all produced by intensive military ballistics research over the last 150 years, but we only use two in rifle ballistic calculations.

 

There is no conversion factor for G7 to G1 BC values of vice-versa - both are standalone and must be used in their appropriate tables, ballistic program, App etc.

 

The biggest difference between them is that the G1 BC is very velocity dependent, so a bullet that starts at over 3,000 fps at the muzzle but goes subsonic at the target at ~ 1,100 fps sees a large range of BC values applying to different parts of its flight, thus making the accuracy of a single figure (or even triple / quadruple as provided by Sierra for different speed bands) relatively poor for a long-range shot. The G7 average BC value only varies by typically 5% or so over a 1,000 yard flight and the big changes of its velocity. Bullet manufacturers who quote G1 values tend to use the highest figure they obtain which may not be very representative of what the buyer uses the bullet for. (Sierra is an exception and its BCs are fairly conservative.)

 

As a result, there's not a huge amount of difference in claculated trajectories, retained velocities, and wind drift at up to ~500 yards range. 600 is the parting of the ways and after there, the G1 results become increasingly inaccurate, usually on the optimistic side. If we take Sierra's 3-value G1 BCs for the 175gn MatchKing at 2,650 fps MV, run it through Sierra Infinity v6, and compare that to the Litz / Berger g7 result, we get

 

 

600 yards

Sierra: .................. 1,672 fps retained vel; 15.7-MOA drop; 5-MOA wind drift in a 10 mph / 90-deg crosswind.

Litz (G7) ............... 1,651 fps ..................... 15.9-MOA ......... 5.1-MOA

 

1,000 yards

Sierra: .................. 1,202 fps retained vel; 38-MOA drop; 9.7-MOA wind drift in a 10 mph / 90-deg crosswind

Litz (G7) ............... 1,118 fps ..................... 39.3-MOA ...... 10.33-MOA

 

The big change here is that Sierra's values say the bullet will be transonic, but still supersonic at 1K, while G7 says it'll be just subsonic (speed of sound = 1,226 fps under standard ballistic conditions). In practice, with a range of 20-30 fps MVs, a 20-shot match will likely see a mix of super and and subsonic shots on the target with this bullet at 1K which will likely give very variable and inconsistent results. The 600 yd values don't vary by enough to bother most people.

 

The G1 / G7 difference at long ranges will be substantially greater with some other makes / models of bullet given their manufacturers' more fanciful BC value claims!

[Lukas_K]

+1 for Laurie´s explanation. If you can get stepped G1 data, the difference is indiscernible in practical application. Did my own real-world experiment a little earlier this year:

 

Comparing combined G1 coefficient with G7:

 

The bullet is .30 Nosler Custom Competition 155grs, which is exact copy of Sierra´s original #2155 Palma bullet. It´s loaded to Mv of 2850fps (only basic F1 Chrony data).

We can get three different G1 coefficients for dropping speed from here: http://www.sierrabullets.com/bullets...ient-rifle.pdf

.450 @ 2600 fps and above
.443 between 1800 and 2600 fps
.417 @ 1800 fps and below

We can use .213 G7 reported by Bryan Litz: http://www.appliedballisticsllc.com/...ALMAbullet.pdf

Now I´ve created 2 ammo profiles in my app, only difference being the ballistic coefficients used for calculation.

I´ve input some average atmospheric conditions and these are the theoretical results:

G1: needs 11.2Mils for 1000m shot and reports transonic at 1025m.
G7: needs 11.8Mils for 1000m shot and reports transonic 925m already.

The difference is not through the roof, but enough to be hit or miss on the first shot.

 

I got the data and the rifle to the range later and reported the following results to the experiment:

 

Both calculations were within .4mils at 960m sight-in (1050yards). We had a realy hot day, over 30°C in the open (+-80-90F); I had to adjust the MV using the Shooters´ muzzle velocity tool -> came out at 898m/s for the G1 and 900m/s for the G7 (2946fps and 2952fps respectively, very close for both in between them).

 

After truing the MV according to the observed hits (and having had confirmed 100m zero first), BOTH G1 and G7 calculations gave spot on with given elevation dope as verified across the course from 1000m down to 600m (1093yds to 656yds).

 

Personal conclusion - garbage in, garbage out. As long as I avoid that and pay attention to details, both systems work to equal precision. The preference to one may best be because of availability of input information, thus giving more options to the shooter... and that´s it.

[Laurie]

For a long range application, the retained terminal velocity is often the crucial factor for cartridges like .223 Rem and .308 Win. The guys shooting 180gn VLDs in 7mm short magnums and .284 Win simply don't have to worry aboutr 1,000 yard velocities, or even for those at 1,200 yards come to that.

 

It's not just the speed of sound / supersonic barrier issue as the problems may start a lot earlier as the bullet enters the transonic zone, ~ 1.2 MACH or 1,351 fps under standard ballistic conditions. This gets into the issue of bullet design as some shapes obviously cope better with such transitions than others - the original #2155 0.308/155gn Sierra Palma MK is obviously a good one, otherwise all these people shooting 1,000 yard matches in 'The Imperial' in a week or two's time would be in trouble with 2,915-2,975 fps MVs in their TR rifles.

 

The US Army did some comprehensive testing of the then commonly used US 173gn .30 FMJBT 'M1' bullet and its Frankford Arsenal match version back in the final days of the .30-06 as the USA's service and primary match cartridge. It came up with 1,226 fps as the key retained velocity at maximum range. Go below that and group dispersion increased even in ideal conditions, and wind changes were found to have a disproportionate effect on trajectories. I don't know if that applies to other .30 boat-tail models, never mind bullets in other calibres, but I suspect strongly that there will be a boundary somewhere around that speed for most bullets. Certainly, no F/TR competitor using 155 / 155.5gn bullets wins or even comes close to winning the F-Class league's annual 1,200 yard match in the Long-Range Challenge weekend at Bisley, held last weekend this year. (From next year, the 1,200 yard stage will be dropped and the 1,100 yarder will be the maximum.)