Drag Function question.

[baldeagle713]

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

[Jeff In TX]

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

[TiroFijo]

"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< vel >1600
and .515 below 1600
This is only a 3.4% variation, obviously a very good fit.

[Jeff In TX]

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

[TiroFijo]

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/eb...ned/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 ]

[Jeff In TX]

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 ]

[Brent]

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?