# Drag Function question.

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by baldeagle713, Mar 6, 2004.

1. ### baldeagle713Well-Known Member

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What drag function do ya use with
1. A VLD bullet (JLK, Burger ect.)
2. A SMK?
3. Horendy VMAX?

2. ### Jeff In TXWell-Known Member

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Bald Eagle,

VLD bullets use a G7 drag model
Flat Base Spire Points use a G6 drag model
Boat tail bullets use a G5 drag model
SMK? I'm guessing is a Sierra Match King, which is a boat tail bullets

One thing to remember is a lot of these bullets straddle the fence as to what they really are (A VLD or BT) and which drag model will work the best.

VMAX is one of those bullets that I've found that straddles the fence between boat tail and/or VLD.

If you know your actual external ballistics, then you can try both to see which one matches your actual data.

Hope it helps

3. ### TiroFijoActive Member

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"VLD bullets use a G7 drag model
Flat Base Spire Points use a G6 drag model
Boat tail bullets use a G5 drag model
SMK? I'm guessing is a Sierra Match King, which is a boat tail bullets"

Not always, and remember most bullets don't match any drag function perfectly.

According to Sierra, after extensive tests the G1 function is the one that fits most of their bullets best (even some of those sleek SMK HPBT bullets).
Since no bullet will correspond exactly with a given drag function, their Infinity program uses the G1 function with a different BC at different velocity ranges.
For example their data with the 190 SMK is:
.533 avove 2100 fps
.525 at 2100&lt; vel &gt;1600
and .515 below 1600
This is only a 3.4% variation, obviously a very good fit.

4. ### Jeff In TXWell-Known Member

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The G1 drag model is usually only accuate out to 500 or 600 yards

The bigger issues I see on published BC’s is using the proper drag model to match the type of bullet you’re shooting. Bullet manufactures are in business to sell more bullets. A lot of shooters buy bullets based on high BC numbers. Bullet manufactures publish their bullets using the G1 drag model. G1 drag model yields higher BC’s numbers, but are very inaccurate for determining exterior ballistic charts for long range shooting.

The G1 drag model is based on a 100+ year-old calculation using a 1 inch 1 pound round nose projectile as its basis. So what the bullet manufactures are really saying is…. any type of bullet regardless of shape can be calculated using the same mathematical model. So if I have a two bullets say a 325 grain .416 barnes X bullet with a published G1 BC of .470 and a 150 grain speer spitzer BT with a published G1 of .472, both leaving the muzzle at 2600 per second they are going to have the same aerodynamic flight characteristics out to a 1000 yards. I think not!

Those are the real issues I see when it comes to BC. Boat tail bullets us a G5 drag model and VLD bullets use a G7 drag model. Both drag models have a lower BC both totally different calculations for determining the bullets exterior ballistic tables.

OK Brent and Michael..... It's your turn! LOL

5. ### TiroFijoActive Member

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Jeff, check what Sierra says, they do test their own bullets at several velocity ranges, and particularly the BCs or their bullets used by the US armed forces (such as the .308" 175 and 190 SMK) were extensively tested with doppler radars:
http://www.exteriorballistics.com/ebexplained/5th/24.cfm

"The G1 drag function is the best standard drag model to use. We have tested several drag functions (G1 for sporting bullets; GL for lead bullets; G5 for boat tail bullets; and G6 for flat base, sharp pointed, fully jacketed bullets). For each drag function we have measured BC values referenced to that function and observed how those BC values change with bullet velocity. We have chosen G1 because the changes in BC values with bullet velocity are least, and because there is a vast database in the literature on BC values referenced to the G1 standard. Also, to our knowledge all projectile manufacturers refer their published BC values to the G1 drag function, which facilitates comparisons among bullets of different calibers, weights, shapes and manufacturers."

BTW, a 300 WM firing a 190 SMK @ 3000 fps, std. cond., goes below 1600 fps at 920 yds. And you won't get a better BC match with ANY drag function than the 3.4% I just mentioned.

In my experience the Infinity trajectory predictions are spot on up to 1000 yds.

Of course for other makers choose the drag funtion that theoretically should fit best, but don't expect a perfect fit.

[ 03-07-2004: Message edited by: TiroFijo ]

6. ### Jeff In TXWell-Known Member

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TiroFijo,

I understand ballistics very well; it’s a bit of a hobby and passion of mine. I shoot Sierra's 168 Gr match kings and Hornady's 168 AMAX out of my .308 to a 1000 yards.

I also understand that no ballistic program or G? drag model is exact. I’ve had many talks with the ballistic technicians at the US Army’s Aberdeen Proving Grounds. Ballistic is what they do for a living. They utilized the lastest and greatest equipment for measuring ballistic including doppler radar. They will tell you that a G1 drag model is only accurate out to 450 to 600 yards depending on the type of bullet. Beyond that, the G1 model is very inaccurate. Those are very short distances for them to measure. They normally work from 700 yards and beyond and the drag models they’ve developed are very accurate at the ranges they test

Also understand, some ballistic programs are written using a modified G1 drag function to massage the data and utilized multiple BC’s at different ranges. My understanding from the Sierra techs I've talked with is Sierra's testing is done in a controlled environment 300-yard indoor tunnel range. Talking with their techs, they don't test their bullets much beyond that. So their G1 drag function will probably work very well at those ranges. I've tried the G1 drag functions for both bullets I shoot. Using the G1 drag function, I’m 31 inches low at 1000 yards to my actual data. That’s not close to being accurate, but my RSI ballistic lab has me within 6 inches of my actual data using the G5 drag model. Not perfect, but very accurate.

Good discussion though. The info we get the better informed we are.

[ 03-07-2004: Message edited by: Jeff In TX ]

7. ### BrentWell-Known Member

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Tiro,
I do understand what you are saying, but doesn't the trajectory predictions depend on which program you use and the algorythims they use for the G1 curve, if you say it's the correct one to use?

8. ### TiroFijoActive Member

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Brent, I really don't know about the algorythims. The G1 funtion is the one the Infinity uses, BUT the program can use the different BCs at different velocities, most programs cannot. There is somewhere in the web a version of the JBM ballistics program altered to use different BC at different velocities, and not surprisingly the results are very similar.

Jeff, Sierra tests their bullets in a 300 yds indoor range, and they mimic the velocity decay by downloading. As you probably know this is not the best way to find out real BCs because the spin rate, yaw angle, precession cycles, etc. are not the same, but I understand the differences are minor. A few bullets like the 175 and 190 SMK were tested in military proving grounds, real life extended range tests with doppler radars, yadda, yadda, and this data is of course known to Sierra and matches very well their Infinity program. I've confirmed it myself up to 1000 yds with good results, if your program is giving 31" error then there is something wrong
I'll post the link to the modified JMB program when I find it, so your can use it to compare with your own data.

9. ### Jeff In TXWell-Known Member

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TiroFijo

Brad's program at JBM uses a very massaged G1 drag model. Brad is extremely knowledgeable about ballistics. He used to have an outstanding ballistic program called JBM On Target but quit selling it back around 96/97 time frame. If you ask him, he will tell you that he uses the same calculations that have come out of Aberdeen proving grounds. When using his on-line program using the G1 drag function, it has been designed/massaged to match the G5 drag model and actual data for the bullets tested by the US Army. Which a lot of times has been the various sierra match king bullets.

Like Brent said, a lot of these programs use a G1 drag model, but many have different mathematical calculations/algorithms. I’ve talked with some of the designers of these programs. Basically they worked with a particular bullet and load they were shooting and came up with a mathematical calculation that matched their actual data very closely. Then they designed and published their program around that data. So if you shoot a bullet that closely resembles the one they used to design their program, you will do very well. But remember they call it a G1 drag function.

Common sense still says that a boat tail bullet, a VLD bullet, flat base spire point and a flat nosed bullets will all have very different aerodynamic flight characteristics at ranges from 400 to 1500 yards. Using a single mathematical calculation designed around one drag model to calculate the exterior ballistic of all those different bullet types is a receipt for failure.

Those of us who are serious about long range shooting spent a lot of time shooting at long ranges. We also keep a very accurate logbook with all of our shooting data. Once you have that, it makes it easy to apply those numbers to a computer ballistic program and if necessary play with your data inputs so that you can closely match your actual data.

10. ### baldeagle713Well-Known Member

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Thank you so very much.
I would be real unhappy if I had a Ground hog in my sights and it got away due to using the wrong setting when caclulating my drop chart.
By the way I saw my first LIVE grong hog yesterday. It is to bad I was on my way to work a MONSTER TRUCK show at the Brice Jorden Center in State Collage Pa. Grave Digger RULES!!!!!

[ 03-07-2004: Message edited by: baldeagle713 ]

11. ### Blaine FieldsWell-Known Member

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<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> Brent, I really don't know about the algorythims. The G1 funtion is the one the Infinity uses, BUT the program can use the different BCs at different velocities, most programs cannot. <HR></BLOCKQUOTE>

The reason that the Infinity uses increasingly smaller BCs is to correct the G1 drag function output. This function will produce drag results that are much lower than actual, hence the need to drop the BC along the trajectory. The fact that Sierra has to patch the trajectory in this way should be demonstration enough that the G1 drag function used by itself is not a good fit for small caliber bullets.

But, as the guys have said, better drag functions use smaller BCs and that is a marketing problem.

12. ### JBMWell-Known Member

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For the most part the military doesn't really use BCs and Drag functions anymore (in talking with Bob McCoy before he died). They use CD versus Mach Number. Of course the ratio of the drag function and BC gives you CD (with some constants and a velocity term thrown in) so mathematically it doesn't really matter which G function you use as long as you have the right BCs (that's the real trick). Of course if you're using 10-25 BCs you might as well use CD and Mach number. As for not being accurate down range, it sounds like the low velocity BCs aren't correct.

My online programs use some old second order curve fits to published G functions. My MPM program doesn't use drag functions at all except in output. My personal code, PM, is a point mass program and it uses linear interpolation into the ARL (used to be BRL) tables of G functions as a function of mach number.

The real utility in drag functions and the reason that you want to find the "right" one that you can limit the number of BCs you have to use. As an extreme example, if you bother to create a drag function for the 300 gn MK, you could use it over the whole velocity range with a BC of 1.0. Of course your drag function is really just the CD as a function of mach number (with the same constants and the velocity term).

Berger sent me some data for their 210 grain VLD. It showed BCs as a function of mach number, velocity and drag function. The G7 drag function varied from 0.316 to 0.338 over the range of 1500-3500 f/s. The G1 BC varied from 0.590 to 0.715.

13. ### MAXWell-Known Member

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Gee, I wish I'd said that! Thanks for the illumination Brad.

FWIW, I think that by the time the average shooter reads/hears the formula for BC the eyes start to glaze, and the marketing types say "Got another one!" In their shoes I'd cast my soul aside and kneel before G1, "Magnum", and "bigger is ALWAYS better." Snake oil by any other name is still snake oil.

What a relief this place is sometimes!

14. ### TiroFijoActive Member

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Thanks JBM, that makes sense.
Do you perhaps remember the site where there is a duplicate of your trajectory program adapter to imput several BC at different velocity ranges?

Now, does anybody thinks you are going to find a better "fit" than the G1 fuction for the 190 SMK (only 3.4% BC variation)? Seems highly unlikely. If you use any other curve you'll have to "patch it" some more