Please help me with some ballistics calculations

TRexF16

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My new Christensen Mesa 6.5 CM is doing great. Even during the barrel break-in phase, two sub 1/2 MOA loads have emerged:
- 120 NBT over 45/H4350 for 3000 FPS
- 139 Scenar over 41.5/H4350 for 2700 FPS
I have run both of these using G7 BC's measured by Bryan Litz, .214 for the 120 BT and .285 for the 139 Scn. I used a program I am used to, the Burris Ballistics to run the trajectory and drift numbers, (out to 800 yards, with a 10 MPH crosswind, and 200 yard zero) and am surprised by one aspect of the results, which I would appreciate some backup by folks familiar with more sophisticated programs. Here's what did NOT surprise me:
- the 120 wins the velocity race out to 650 yards and then the 139 carries greater velocity beyond that.
- the 120 wins the energy race to 250 yards, and then the 139 carries greater energy beyond that.
- the 120 always wins drop, having an 8.5" advantage at 600 yards. The advantage increases to 16" at 800 yards
and here's what surprised me:
- the 139 ALWAYS wins drift, and its advantage builds to 4" at 600 yards, increasing to 8" at 800 yards.
I had expected, given the 300 FPS MV advantage, that the 120 would have less drift over the first couple hundred yards. This program shows the 120 has a 20 msec. (.020 sec) TOF advantage to 200 yards. That's pretty significant. Yet the program still shows the 139 having a 0.3" drift advantage at 200 yards.
Does this sound right?

Much appreciated,
Rex
 

.gacton

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Rex, I wish I could give a long Litz-type explanation. But, I’m not that smart. 😁 How do the loads print on target? Do your groups show the wind drift your calculations come up with? I’ve found the ballistic calculators get me close, better than a W.A.G.. But I always try to verify what my chosen app tells me. Hope that helps. Hell, I hope it makes sense. I’ve only had one cup of coffee!
 

TRexF16

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I only have 53 rounds down the barrel so far, and all shooting has been at 100 yards. Interestingly, those two loads I mentioned hit just about the same POI at 100, which is both surprising and encouraging to me. Winds have been pretty calm for all the range trips so far.
This Mesa is turning out to be a really nice rifle. Pretty much everything I have shot has been MOA or less, but I've shot 16 rounds of the 120 NBT load and those are shooting really tight. Just tried two groups with the Scenars this week and they are both in the 1/2 MOA range too, so high hopes developing so far.

Thanks,
Rex
 

ShtrRdy

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It's been a while since I read Bryan Litz's book, "Applied Ballistics for Long Range Shooting", but in the chapter on wind drift he talks about Dwell Time. Dwell Time is basically the actual TOF of the bullet minus the TOF in a vacuum. Then take this Dwell Time and multiply it by the wind speed in feet/sec and you get bullet drift in feet. Then you convert feet to inches.
 

BallisticsGuy

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While, yes, there's more volume in a larger bullet which means there's more surface area for wind to affect, that increased surface area is proportionally much smaller than the proportional increase in mass. Increasing mass means more wind velocity or more time is required to net the same deflection through momentum transfer. It's not hard to conceptualize but I guess it can be counterintuitive to the uninitiated.

This is why in the long range classes I teach, we spend a lot of time on understanding how bullets and air behave in each other's company. Any idiot can point a rifle and twist a knob. Knowing what knobs to twist is easy. Knowing exactly why you're twisting knobs, that's where the value lay.
 

TRexF16

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I just re-ran the comparison using the JBM ballistics online tool and the results were similar. The 139 Scenar had less drift throughout the entire range, beginning at the muzzle. The 120 NBT never had a drift advantage even with its 300 FPS muzzle velocity advantage. I did not expect that.

Live and learn!
Rex
 

Jeffpatton00

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While, yes, there's more volume in a larger bullet which means there's more surface area for wind to affect, that increased surface area is proportionally much smaller than the proportional increase in mass. Increasing mass means more wind velocity or more time is required to net the same deflection through momentum transfer. It's not hard to conceptualize but I guess it can be counterintuitive to the uninitiated.

This is why in the long range classes I teach, we spend a lot of time on understanding how bullets and air behave in each other's company. Any idiot can point a rifle and twist a knob. Knowing what knobs to twist is easy. Knowing exactly why you're twisting knobs, that's where the value lay.
Ballistics Guy, it doesn't look like I'm able to message you directly. Would you shoot me a msg? I'd like to discuss your ballistics class. thx
 

entoptics

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While, yes, there's more volume in a larger bullet which means there's more surface area for wind to affect, that increased surface area is proportionally much smaller than the proportional increase in mass...
Unless I'm making a math mistake somewhere, I don't think this is true...

Assume a cylindrical projectile (not a pointy bullet, but concept is the same) and a density of lead of 11.29 g/cm3. The frontal area of the cylinder is fixed since the bullet diameter is fixed, so...

120 grain cylinder
Side area = 0.3269 cm2

139 grain cylinder
Side area - 0.3787 cm2

Mass ratio - 139/120 = 115.8%
Side area ratio - 0.3787/0.3269 = 115.8%

I feel like I'm wrong, but triple checked my math, so I'm inclined to think something else is at play here...

Obviously, for forward motion, the ratio does change. 1 to 1 frontal area ratio, vs 1 to 1.158 mass ratio, so more mass per drag makes sense in drop data, but not wind.

🤔
 

Teri Anne

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While, yes, there's more volume in a larger bullet which means there's more surface area for wind to affect, that increased surface area is proportionally much smaller than the proportional increase in mass. Increasing mass means more wind velocity or more time is required to net the same deflection through momentum transfer. It's not hard to conceptualize but I guess it can be counterintuitive to the uninitiated.

This is why in the long range classes I teach, we spend a lot of time on understanding how bullets and air behave in each other's company. Any idiot can point a rifle and twist a knob. Knowing what knobs to twist is easy. Knowing exactly why you're twisting knobs, that's where the value lay.
And this is why old military snipers and long range shooters are much more intuitive to what is going on around them and between them and the target that the new people who are relying on gadgets and giggles to do the calculations for them. Old time shooters estimate the wind using trees, grass and mirage to name a few to do a by guess and by golly as to what the bullet is going to do between here and there. If on a range and the range flag on the firing line is showing wind at 90 degrees at 10 mph and the range flag down by the targets is showing 270 degrees at 20 mph which do you believe? There is more to long range shooting than twisting dials and gauging bullet drop.
 

Tulsa Reiner

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Unless I'm making a math mistake somewhere, I don't think this is true...

Assume a cylindrical projectile (not a pointy bullet, but concept is the same) and a density of lead of 11.29 g/cm3. The frontal area of the cylinder is fixed since the bullet diameter is fixed, so...

120 grain cylinder
Side area = 0.3269 cm2

139 grain cylinder
Side area - 0.3787 cm2

Mass ratio - 139/120 = 115.8%
Side area ratio - 0.3787/0.3269 = 115.8%

I feel like I'm wrong, but triple checked my math, so I'm inclined to think something else is at play here...

Obviously, for forward motion, the ratio does change. 1 to 1 frontal area ratio, vs 1 to 1.158 mass ratio, so more mass per drag makes sense in drop data, but not wind.
You've got me wondering too.
I hope Ballistics Guy can give us some insight.
 
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