Range Report Correct Native Drag Model

[Saito]

What would be the correct Native Drag Model for a .308dia 208 grain Hornady A-max when using the Ballistic FTE program on the Ipod Touch? I'm currently using G1, I know from experience that G7(VLD boat tail) is not correct. G5(Low Base Boat Tail) looks like it may be correct. I've only tried G7 and G1, and so far G1 seems closest to actual trajectory seen while engaging targets. Thanks in advance.

 

[BryanLitz]

Re: Correct Native Drag Model

Saito,
Please don't take offense to the following, you never know the experience level of others in these forums...
When using BC's referenced to standards other than G1, you need to enter a different numeric value than given by the manufacturer. All of the BC's given by manufacturers are referenced to G1. If you select G7 in your program and enter the value given by the manufacturer (or close to it), the results will be unrealistic because the inputs are inconsistent.

I've measured the G7 BC of the Hornady 208 Amax to be 0.324. This is quite a bit different than the G1 BC, but it's right for the G7 standard. Try that number in your program with G7 selected and see what you get. Since the 208 Amax is a secant ogive boat tail bullet, it does match the G7 standard quite well, but you have to give it the correct numeric G7 BC (0.324).

Again, I don't mean to insult your intelligence if you know all this, but wanted to put it out there in case.

-Bryan

 

[Saito]

Re: Correct Native Drag Model

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">Saito,
Please don't take offense to the following, you never know the experience level of others in these forums...
When using BC's referenced to standards other than G1, you need to enter a different numeric value than given by the manufacturer. All of the BC's given by manufacturers are referenced to G1. If you select G7 in your program and enter the value given by the manufacturer (or close to it), the results will be unrealistic because the inputs are inconsistent.

I've measured the G7 BC of the Hornady 208 Amax to be 0.324. This is quite a bit different than the G1 BC, but it's right for the G7 standard. Try that number in your program with G7 selected and see what you get. Since the 208 Amax is a secant ogive boat tail bullet, it does match the G7 standard quite well, but you have to give it the correct numeric G7 BC (0.324).

Again, I don't mean to insult your intelligence if you know all this, but wanted to put it out there in case.

-Bryan </div></div>

Bryan,

No offense taken, funny though, I once held an Intelligence Specialist rating. Thank you, I knew the answer was out there. If I'm not learning, I'm bored. I love this site, there's plenty of experience, a little humor and most people are incredibly helpful.

As the G7 Drag Model description best fit the bullet I was using I punched those #s in. They were way off. I'll have to look and see if I can modify the #s in the program(most things are pre-programed; obviously off of the G1 model). To say the outputs were unrealistic would be an understatement. It was telling me my bullet was still doing 2000fps @ 2K.

I noticed that the BC of the given bullet in the G7 model is exactly half of what it is in G1(as far as the Hornady goes). Would you say that is a standard? I noticed your signature line. I shoot 210 grain Berger VLDs as well. Is the G7 BC of the 210 VLD .3155 or .312? Just thinking.

Thanks again for the info.

 

[BryanLitz]

Re: Correct Native Drag Model

Saito,
Glad to help.
It's a common mistake for users of ballistics programs to choose a different drag model like G7, but then enter a BC that's referenced to G1. As you found, the results are non-physical.

I trust your predicted trajectory is a better match to real world when you use a G7 BC of 0.324?

The G7 BC of the 210 VLD is 0.315. There is a loose correlation between G1 and G7 BC's, but I advise against a blanket application of the conversion. The biggest reason is because the conversion factor depends on what velocity the G1 BC is based on to start with. If you want anymore G7 BC's for Berger bullets, don't hesitate to ask. I've also got limited data on other brands as well that was collected using the same test procedure that's repeatable within +/- 1%.

Take care,
-Bryan

 

[Saito]

Re: Correct Native Drag Model

Got all of the #s punched into the program and saved. There are slight differences as opposed to the G1 model, especially at distance. I'm glad I got that figured out. Being able to trust your gear is a huge thing for me.

I would like to know the G7 BC of the 190 VLD as I have a pile of those as well. Would you think that Moly coating would impact the flight characteristics of the bullets? I.E. cut down on the friction of the bullet cutting through the air? I realize what it does in the barrel, just wondering if it has advantages over a naked round in flight.

Thanks again,
Nick

 

[BryanLitz]

Re: Correct Native Drag Model

Nick,
The G7 BC for the 190 VLD is <span style="font-weight: bold">0.291</span>.
Moly will not affect the BC, but it will affect your muzzle velocity. You'll have to re-chrono your moly loads.
-Bryan