G1 or G7?

[ruger_steve]

Hi all.

I am having a play with my Bushnell app and in programming my rounds into it to work out trajectories and the like, after selecting Lapua Scenar .308 HPBT 167gn it has asked me if it is a G1 or G7 bullet.

I have had a look on the web but cant really seem to find anything (that makes sense anyway) of the difference other than a few images that suggest a G1 is a straight round and a G7 is, well, boat tail shaped. Is this correct or is there more to it?

I definitely have HPBT so is that definitely a G7?

Thanks, Steve.

[WelshShooter]

That is also my understanding of G1 and G7. If you're using ballistic software to calculate drop/velocity over distance you'd be best to use the drag factor which bent represents your bullet type, in this case it's G7. However, there are a fair few manufacturers who only state G1 and not G7 so in this respect it can still be useful to compare the two G1 values between two bullets of similar profile (eg SMK and Hornady A Max).

[ruger_steve]

That is also my understanding of G1 and G7. If you're using ballistic software to calculate drop/velocity over distance you'd be best to use the drag factor which bent represents your bullet type, in this case it's G7. However, there are a fair few manufacturers who only state G1 and not G7 so in this respect it can still be useful to compare the two G1 values between two bullets of similar profile (eg SMK and Hornady A Max).

Thank you. Ive got something right then. ha If anyone has any differing opinions, please say.

[mag41uk]

https://www.lapua.com/bullets/scenar/

Have a look at JBM ballistic software.

[Laurie]

What you must understand in attempting to sort out 'standard BCs' (ie as published, not custom created BC values) is that they all start with a 'reference projectile' and the BC system used compares the subject bullet against it. The 'reference projectiles' are all called G-something, each a different shape.

Being precise, it isn't BC that is compared to the G-something equivalent, it is the 'form factor' abbreviated to i-something, the something being that of the 'reference projectile' and its-whatever always has the value of 1.000. This is a drag-based metric, so if a sample bullet has an i1 or 17 value of 1.000, it generates the same drag in flight as the reference design. If lower, it is better; if higher it generates more drag. The other factor in creating BC is the SD or sectional density which is mass to calibre ratio and where any and every projectile in a given weight and calibre has the same SD value irrespective of its shape and resulting aerodynamic efficiency.

Combine the two and you get the BC, but from what I've said it's obvious that the key metric is the 'form factor' for any two bullets in a calibre especially if they're the same weight as this is the thing that sorts those with the shape of the proverbial brick-built kazi out from those of the super-low-drag streamlined type. .......... and as stated it all starts with a fixed reference design which the subject is compared against.

So, if you build your house on such a comparison, you want to compare like with like, so you want the G-whatever reference base to have as close a shape to the bullets you use as possible.

The G1 is an old (late 19th century) flat-base short-radius round nose projectile, originally that of the one or two-pounder British 'Pom-Pom' shell. The G7 reference is that of a modern US designed long-range artillery shell whose shape (or 'form') happens to be very close to that of a modern streamlined low-drag, boat-tailed rifle bullet.

The G1 form is useful if working with the ballistics of early RNFMJ military rifle bullets such as those in 303 calibre prior to the adoption of the pointed Mk VII in 1910. It is ideal for a typical 40gn 0.223" solid lead .22RF bullet whose shape is nearly identical to that of the G1, the heeled-base aside. Otherwise you want an average G7 BC value. Bryan Litz has tested and published the average BCs of some 950 or so bullets in his book Ballistic Performance of Rifle Bullets 3rd edition and this is the source of much data used including most of that in JBM and other apps. The larger military outfits and a few bullet companies now have Doppler radar systems that track a bullet's flight and speed over its entire length and provide even finer data, but Litz is good enough for mots purposes ELR aside where custom form factors are being developed and used in ballistics apps - unmarked, no-sighter shots at targets two miles away as in the annual Ko2M comp requires yet finer base data before attempting to measure and apply ambient environmental factors.

The issue is very distance-determined. Shoot at 300 yards, even 4/500 and it really doesn't make much difference which metric is used. A 308 bullet shot at 1,000 at 308 Win velocities though sees large differences in results, potentially crucially. The problem with the G1 with a lower drag design is that its values are very speed related - ie drag changes alongside bullet speed changes. Sierra gets around that by quoting multiple G1 BC values in speed brackets. To use the Sierra multi-BC model, an appropriate ballistics solver program has to be used that automatically changes BC at stages in a long-distance flight, and even then it is an approximation.

To take the 90gn 0.224" MK HPBT as an example, Sierra quotes five BCs - 0.504 at 2,200 fps and above and reducing in steps to 0.305 at below 1,575 fps. Many manufacturers - pre Litz blowing their claims out of the water - used to take the highest value which might only be obtained at say 3,500 fps and above and quote it as THE BC VALUE for their product. The reason for the need for this is that bullets such as the 90gn MK use streamlined shapes and the model is attempting to compare it against a simpler much higher drag model, ie hammering a square peg into a round hole.

Litz gives the average G7 BC for the 90gn 224 SMK as 0.257 average. That is is only a little above half that of the G1 value is unimportant as the two forms use very different scales and aren't comparable. Average G7works for most purposes when the sample is being compared to the more suitable G7 reference design. (This Sierra bullet has an i7 value of 0.999, so is nearly identical drag-wise to the reference) there is little change in the effective BC over a long flight and large speed change, say 4 or 5% over 1,000 yards, much less from 3,000 fps down to 1,500 fps. Once subsonic though, all the rules change again and standard BCs are unreliable. Also as the bullet drops to 1.2 MACH or below it enters 'transsonic' flight and some designs can see much increased drag values through turbulence around them especially below 1.1 MACH whilst others are relatively unaffected.

Taking Sierra's popular 175gn 0.308" MK in a typical 308 Win sporter or tactical rifle at around 2,650 fps MV, let's look at results for a 1,000 yard flight under three scenarios:

1) the old approach some companies (not Sierra) used quoting the single best G1 BC value only, in this case 0.505.

At 1,000 yards the solver says it's doing 1,229 fps (just under 1.1 MACH but avoiding the worst of transsonic effects) and moves 98 inches laterally in a 10 mph 90-deg crosswind. It needs + 37.6-MOA elevation on the scope or sights on a 100 yard zero.

2) Sierra's banded BCs. (This bullet only has three values given to it not covering a great range so the effects aren't as marked as many such as the 224 90gn SMK mentioned.)

At 1,000 Sierra says the expected retained velocity is now 1,203 fps, wind drift is 103 inches and + 38.3-MOA elevation is needed.

3) Litz obtained (through actual experimentation and recording downrange speeds) average G7 BC of 0.243.

At 1,000 Litz's program says the expected retained velocity drops to a subsonic 1,115 fps, wind drift increases to 108.6 inches and elevation needed is 39.4-MOA.

Having shot this bullet at a slightly lower MV into a violent and angle-changing headwind in low temperatures in spring 2008 or 2009 in a 1,000 yard F-Class comp at Blair Atholl, the Litz version appears much closer to reality than the others even without any possible (likely) transsonic / subsonic barrier effects on bullet stability. The 175gn SMK is actually a very well behaved low velocity / transsonic performer, but even so, US Army work on the old 173gn M1 FMJBT bullet showed that terminal speeds below ~1,225 fps (1.1 MACH) doubled group size and added around 50% to wind drift values.

[Laurie]

As a picture is worth a lot of words, here is a piece from Accurate Shooter with the two G variants drawings and some more information.

http://bulletin.accurateshooter.com/?s= ... mit=Search

[ruger_steve]

As a picture is worth a lot of words, here is a piece from Accurate Shooter with the two G variants drawings and some more information.

http://bulletin.accurateshooter.com/?s= ... mit=Search

Brilliant, this is the picture I was looking at.

Thanks all for all the info. Definitely got G7's.

Thanks.