BC Method of Doping Wind - Brackets?

[kmckinnon]

I'm reading Frank Galli's book where in it's described using the first number of your bullets BC to determine the MPH of your rifle. The example, a 308 using a bullet with a BC of 485 to 496 is a "4 MPH rifle". One may then work in multiples of 4 mph (and not the traditional/usual 10 mph) to dope wind. I get this.

But I got confused when in a later it is said "I just need to know my brackets then do simple addition in my head." The example at 600 yards, with a 4 mph wind is the first bracket, so that means a .6 Mil hold. Makes sense to me. But it goes on to say "If the wind is 12 mph, I am in bracket 2, so the wind hold is 1.2 Mils." While 2 x .6 Mil = 1.2 Mils, I don't get 12 mph being in bracket 2. If I have a 4 mph rifle wouldn't a 12 mph wind put me in bracket 3 (12/4 = 3) ? And that's followed by an example 18 mph wind being bracket 3 with a 1.8 Mil hold (3 x .6 = 1.8). Again, I get how I get to 1.8 but if I'm using a 4 mph multiple wouldn't 18 mph put me in bracket 4 (18/4 = 4 and change).

The last example provided describes engaging an 8 mph wind with a 308 rifle at a 400 yard target and the WTF suggested dial of .8 Mils for the first shot. 400 = .4 Mils so using a 4 MPH rifle I'd need 8/4 = 2 giving me 2 x .4 Mils = .8 Mils. Is an 8 mph wind putting me in bracket 2?

I'm working with a 308/4 MPH rifle but not understanding brackets, or at least the size of brackets. If an 8 mph wind gets me bracket 2, how does a 12 mph wind get me bracket 2? And an 18 mph wind get me bracket 3?

Thanks in advance for helping this total newb!

 

[kmckinnon]

Now if the example using a 12 mph wind putting me in bracket 2 was using a 6.5 CM with irs bullets BC in the 5xx range then I could see 12 mph putting me in bracket 2 (12/5 = 2 and change) and an 18 mph wind getting me to bracket 3 (18/5 = 3 and change).

Perhaps my confusion comes from thinking the examples were for a 308/4mph rifle used in some examples while this particular example was for a 6.5CM/5mph rifle?

This discussion and examples are in Precision Rifle Marksmanship: The Fundamentals, pages 178-179.

 

[PRCslut]

Do you know the wind deflection formula

 

[HABU]

I read this last night. I reread it two more times because there is a disconnect somewhere. I assumed it was with me, but for the life of me I can't put all the pieces together.

 

[DeftSystems]

Can you post a screenshot or link to this small part of the book so we can understand this? I googled and tried AI it but didn't come back with a satisfactory explanation. I'll say that it's common to have a wind formula setup where your correction = speed*yardline / coefficient, and I have seen and used those in the past.

I prefer the practice of building a table with a quality ballistics calculator and then going off of that. You just start with much more accuracy. You can also build a custom formula for your particular setup if you are slightly handy with Excel. I'll even make an Excel file for you if you like, just give me your bullet info.

 

[usafa77]

Forget the names of the brackets. You understand the 4mph part, that's all you need to know about the mph gun number. It's a good rule of thumb.