G1 and G7 or CDM?

PhilM · June 7, 2017

Hi folks,

I know a few of you are budding ballisticians, so I thought I'd ask.

I am considering a Kestrel 5700 and can't decide between the Sportsman, which uses G1/G7 ballistic coefficients, and the Elite, which has the option to use Applied Ballistics's range of Custom Drag Models.

I know there are other feature differences between the two I am taking into account, but I don't know if there are any real advantage in using CDMs?

Any thoughts?

Cheers

borbal · June 10, 2017

If you are intending to use sharp pointed bullets, then no. You can do very well with G1 and G7 ballistic coefficients. But if you are intending to use blunt bullets, like hunting bullets with hemispherical heads, then these are very badly modelled when comparing them to the G1 or G7 standard projectile. I don't know what the range of Custom Drag Models is for the Kestral 5700 Elite or if it includes blunt bullets and round balls. You can see more about this on my website, http://www.geoffrey-kolbe.com/trajectory.htm where clicking on the different standard projectiles gives a discussion about the drag.

Dustyman · June 11, 2017

I cAnt seem to get link to work , is it correct chap or is it

Me ? Cheers

gbal · June 11, 2017

If you are an average long range shooter staying within supersonic ranges,then accurate G7 BCs are enough to keep you on target.

If you shoot into transonic ranges and/or if you need your first shot on target (no sighters) then the accuracy you get from Applied Ballistics solver running live fire based CDMs is unmatched.(B.Litz-largely responsible for those accurate G7s and CDMs and AB software ).

((Why? OK It's rather complex,and the basic concept is aerodynamic drag...but it can be made into friendly shooter speak.Lets get the equation out of the way-don't struggle with it...it may look complex but is actually quite simple as these things go...

You canjust blink and skip to READ ON if this looks formidable...

Aerodynamic Drag on a bullet = qSCd (q is dynamic pressure,S is frontal area,CD is drag coefficient).No real problems....as equations go,it's pretty basic:

Drag=(I/2 times .002377 times Velocity squared) times 3.142 times calibre/24 squared) times Cd........

AH!! ....Cd is the fly in this ointment)

READ ON:....The Cd has to be measured by lots of live firing for each bullet,to get that bullet's unique drag curve,from which all the ballistic goodies are derived (G7 is a one standard size 'nearly fits all' -so fairly easy to get data,that will be quite close for most modern boat tail,pointed bullets..the G& model form).

The idea of standard drag curves is generally known-the G1 Is essentially a flat base,cylindrical sides,and relatively rounded (at least not pointy) nose-22rf is a fair approximation,though a tad rounded;this is a poor 'standard' model for most current bullets (though can work for some flat based,at short range).

Much better is the Standard G7-boat tail,cylindrical body and pointy nose...it's quite like many modern bullets,but it's not identical to (m)any of them-think for example of differnt noses-VLD has a very low dragthin poined nose,then there are various ogive/tangent and hybrid mixes,and boat tail differences.

But properly calculated G7 BCs are rather good,and Litz has produced these for a lot of commonly used bullets.A big improvement,and better ballistic solvers can use the G7s. Real progress....but the G7 is still a standard shape,and not quite correct for individual bullets,so not the optimum.

Enter the CDM concept (Custom Drag Model)-near perfect for the appropriate bullet.

It took time to get going,because CDMs are not easy to measure/establish.The standard G1 was readily computable by first military and then commercial modern small arms ammo users/makers. The legacy of using the G1 is considerable,books use them,early ballistic solvers were limited to them...But the Litz derived G7s were a big step up,and that library of G7s now exists,and there are shooter available ballistic solvers and software that can handle G7 (if it doesn't work rather well,the error is in the faulty shooter input,or the need for rather extreme precision).

OK-use CDM-well,yes now we can,but it has taken a very large amount of work to get a CDM for a decent range of bullets. LItz/Applied Ballistics has now done this very large amount of live firing,and the extensive computations to produce CDMs for individual specified long range bullets.

And,OP,there is hardware/software readily available for the point mass solvers that now exist that can use all this (all the older Pejsa ones can't- and much mischief arises when such are 'bent' to fit very limited live fire data-three shots at 600 just won't do). The CDM ones don't need live firing data-it's been done in the CDM and these include AB Analytics software,Kestrel 5700 with AB,as per your OP,maybe some smart phone apps coming..

SO,if state of the science precision prediction of long range shots is your need,CDM gives the best shooting solutions,with an unrivalled combination of ballistic solver and AB's CDM data.

HOW accurate is it-here is sample 30 cal Berger 175 SMK @ 2570fps: Actual drop/predicted drop in mils/error in inches:

1000y. 11.2/11.36/5.8

1200y. 16.6/16.55/2.2

Note the errror remains quite small,and is not linear-it isn't simply more as distance increases.**..( explaining that gets complex!!) and there is about a .1mil error ( 4" +/_) in the observed data due to dispersions in the observed groups. These are the results of raw,untrued/not fiddled with,first shot cold bore accuracy from AB solver and CDM. The CDM does especially well in the transonic ranges,which are the most problematic for the G1 esp and even G7 predictions-as many shooters have found!

TAKE AWAY:as opening statements-if you want/need the best possible shooting solutions for long range,CDM provides this;though G7 isn't shabby at all for average use. Remember poor shooter input,poor output but CDM/G7 will at least remove most computational mismatches More buck,more bang-your choice.Good shooting.

gbal

** Brozovitch data from live fire,raw first shot accuaracy using AP ballistics,and CDM for 338 Berger Hybrids @3198 fps:

800y Prediction. 11.4 moa. Error. 0.1"

1200y. " 21.9 moa. " 4.0"

1600y. " 35.1 moa. " 1.8"

1773y. " 42.2moa. " 1.8"

Hmmm..... Sub 4" error beyond a mile....first round,predictive precision/accuracy.

Ding Dong,Top Gong.....but don't forget to factor in the precision rifle,ammo,shooter skill ....and no wind.

M99 · June 11, 2017

The Elite - for the simple reason you can store more profiles - it is worth the extra for this alone.

Make sure you get the liNk version.

Incidentally the CDMs are great - make a lot of difference. I use mine on a regular basis out to in excess of 3k

PhilM · June 21, 2017

Thanks guys for the comprehensive replies.

I will be using mainly sharp pointed/polymer tipped bullets, no need to blunt nosed ones, but I will be shooting to extended distances.

My 'normal' shoot is 900m, and I fancy pushing things beyond there to 1200+, so perhaps the CDMs will make a difference.

Based on the £130ish difference between the Sportsman and Elite, I think the other features may have swung it, but now CDMs seem like they have their place in addition to G1/G7, I think I'll swing for one of those!