Advanced Marksmanship Question on Rifle MPH/Wind topic

[572]

I hope this question is posted in the right place. I've got a basic understanding on the whole Rifle MPH/Wind thing for determining reticle/dialing holds for wind values but I have several questions. I am using a Vortex Razor Gen 2 MIL scope with .1 MIL adjustments. I'm shooting 6.5 Creedmoor Hornady 147gr ELD-M with a G1 BC of .697

1.) Why is the G1 drag model used for this nifty trick?
2.) If the BC is .697 do I round up at all being so close to .7 to be a 7mph gun and why?
3.) Given I don't round and I have a "6mph gun" why is it considered a 6mph gun when I'm adjusting the wind to a different wind speed number to make the 1/10th MILs line up with the hundred yds? Especially given that the 7mph wind value is what lines up in my solver.
4.) How much rounding is considered with lining up the 1/10th MILs per hundred yards and is it better to round up or down?
5.) Having 6.5CM is it better to find a wind speed to make the 1/10th MILs line up at a distance farther than 600yd?
5.) Does the G7 drag model also work in any way when finding wind speed numbers to come out to .1mil per hundred yards?

 

[Deleted member 113831]

The reason you use the G1 number is just to give you an approximate starting point. It may or may not end up being the right number, but it will be close.

The reason the G1 number works is a fortunate coincidence. G7 numbers are roughly 1/2 the G1 number. Take your whole G7 number and double it (G7 .250 x 2 = .500). Then use the first number of the result.

Altitude and muzzle velocity have an effect.

A G1 .500 BC bullet at 2,000ft elev. @2,800fps is a 5mph gun. That same bullet at 3,000fps is a 6mph, that same bullet at 2,600fps is a 4mph.

A 5mph at 6,000ft elev. is a 6mph. Same bullet at sea level is 4.5mph.

+/- 200fps = 1mph
+/- 4,000 ft elev. = 1mph

 

[572]

Thanks so much for the info. Any insight into rounding?

 

[Steel head]

I always round up.
.630 I’ll call 6 but my Berger 190’s with a .751 definitely working with an 8.

 

[Deleted member 113831]

I always round up.
.630 I’ll call 6 but my Berger 190’s with a .751 definitely working with an 8.

How fast you pushing them?

 

[Deleted member 113831]

Thanks so much for the info. Any insight into rounding?

I line it up to within 0.1mil at 1,000 yards and accept whatever small difference there may be at the closer distances. This might mean anywhere between 0.9 - 1.1mil at 1K, whatever makes it line up best. Some prefer to line it up at 600 or 800 yards. It all depends on your preference.

I don't shoot PRS, so I don't have to worry about whatever retarded scenario I might face at some match. It is basically an academic exercise for me these days. My time of climbing mount motherfucker after an animal, or thinking that I might want to do any snipery shit are over.

I do like stretching to 1 mile and out to 2,400 yards with a couple of my bigger guns, but taking my 308 to 1,200 yards gives me just as much satisfaction and challenge.

 

[572]

My problem is I’m limited to 200yd in MA. I have one place to go at an old military base that has 800 or 900 but until COVID is done I want to get all my numbers done in advance so I know my ballpark. I’m not a competition person but I’d watch a PRS just to see it in action and not on YouTube.

 

[Steel head]

How fast you pushing them?

The 190’s?
A quite mild 2850 fps.

 

[Jscb1b]

I'm reading Frank's book. On page 179 he is talking about a 4mph rifle. I understand that at 600 yards and a 4 mph wind it is a .6 mil hold and in the first bracket. How did he figure 12 mph to be the second bracket and 18 mph to be the third bracket?

 

[Steel head]

I'm reading Frank's book. On page 179 he is talking about a 4mph rifle. I understand that at 600 yards and a 4 mph wind it is a .6 mil hold and in the first bracket. How did he figure 12 mph to be the second bracket and 18 mph to be the third bracket?

12 is 3.0 x Your 4 mph rifle so 3x .6 = a 1.8 hold at 600
18 is 4.5x your 4 mph rifle so 4.5x .6 = a 2.7 hold at 600

 

[Deleted member 113831]

I'm reading Frank's book. On page 179 he is talking about a 4mph rifle. I understand that at 600 yards and a 4 mph wind it is a .6 mil hold and in the first bracket. How did he figure 12 mph to be the second bracket and 18 mph to be the third bracket?

I haven't read the book, but given what you have said, 2nd bracket should be 8mph for 1.2mil @ 600 yards, and the third bracket should be 12mph for 1.8mil @ 600 yards.

 

[Deleted member 113831]

The 190’s?
A quite mild 2850 fps.

BC is .751, and you are pushing them just a bit faster than 2,800fps. So yeah, it makes sense they would be very close to an "8".

 

[Jscb1b]

I haven't read the book, but given what you have said, 2nd bracket should be 8mph for 1.2mil @ 600 yards, and the third bracket should be 12mph for 1.8mil @ 600 yards.

That is what I was thinking too.

 

[_Raining]

Thanks so much for the info. Any insight into rounding?

It is probably easier to just use a ballistic calculator and increase the wind by 1 MPH till you get something like
.1 @ 100 yards
.2 @ 200 yards
.
.
.
.8 @ 800 yards