Fat ain't where it's at...it's BC...

[murf]

cal30 sniper,

a lighter bullet will have less surface area affected by the wind than a heavier bullet of the same bc.

wind, no matter which direction or velocity, will have the same effect on bullets with the same ballistic coefficient, regardless.

murf

 

[Searcher4851]

I'm no expert on this stuff by an means, but doesn't a heavier bullet experience less wind deflection due to inertia?

 

[murf]

searcher4851,

if the bullets had the same surface area, sure. but bullets with the same bc don't. the heavier bullet will have the greater surface area.

kind of like a bigger sailboat having bigger sails so it can go just as fast as the smaller boat.

murf

 

[cal30_sniper]

A "heavier bullet" can easily go just as fast or faster than a "lighter bullet." It all depends on what cartridge you're using. A 300 Win Mag is going to launch a much heavier bullet faster than a much lighter bullet shot out of a .243. What is being discussed here, is if the .243 and .300 bullet were the same BC, and started at the same velocity, would they always arrive in exactly the same place?

 

[cal30_sniper]

cal30 sniper,

a lighter bullet will have less surface area affected by the wind than a heavier bullet of the same bc.

wind, no matter which direction or velocity, will have the same effect on bullets with the same ballistic coefficient, regardless.

murf

I'm still struggling with this statement in my mind. I'm going to have to go mull it over for a bit...

Initial thoughts though. Surface area does not always have the same correlation with weight for a ballistic coefficient. Surface area from directly head on is directly proportional to diameter of the bullet squared. However, from any other angle, the shape of the bullet, and its density, are going to change its surface area relative to its mass or sectional density. I'd say that in any situation I can think of, the heavier bullet should have a larger area than a lighter bullet, but it definitely wouldn't be a 1:1 ratio.

Also, since bullets can be filled with varying substances and covered with different jackets, tipped with different polymers, or left hollow in the nose, a bullet of the same weight and caliber (thus the same sectional density), with the same coefficient of drag, could have the same ballistic coefficient as another bullet but a different shape profile. That would result in differing surface areas experiencing air for anything other than flight with a zero-value wind. The different surface areas would cause the ballistic coefficient to change at varying rates for the two bullets.

However, a BC is calculated for air going straight over the bullet, hence why the diameter of the bullet squared is used as the critical area. At any other angle other than directly forward, the BC is going to change. Two bullets may have the same sectional density and coefficient of drag from dead on, but if they angle sideways and aren't exactly the same shape, that would change, if only slightly, correct?



Thoughts?

 

[Auto426]

What is being discussed here, is if the .243 and .300 bullet were the same BC, and started at the same velocity, would they always arrive in exactly the same place?

I think that's what the OP is trying to argue, but it seems to me that's not what the post the OP quoted was trying to argue. The quoted post seems to be more along the lines of bigger bullets make bigger holes, not so much about how they fly through the air.

Then again I didn't read the thread that post came from, so I don't really know.

 

[helotaxi]

What is being discussed here, is if the .243 and .300 bullet were the same BC, and started at the same velocity, would they always arrive in exactly the same place?

According to Bryan Litz, who is an aerospace engineer, a very accomplish long range shooter and the chief ballistic guru for Berger Bullets, the answer is "yes".

It doesn't matter if you're talking gusts, steady wind, eddies, or whatever, the BC determines how the bullet reacts. Especially when you get into long rage bullets better described by the G7 model, the variability with velocity is accounted for very well. In fact the G7 model accounts for the change in drag coefficient with velocity as part of the model. Bullet mass doesn't play a part in the variability at all.

As far as the side area of the bullet, it is totally irrelevant. The bullet will weathervane into the wind, even with transient gusts. You have to consider that the amount we're talking about here is fractions of a degree since the amount that it yaws is determined by the ratio of the cross wind component to the bullet velocity. So for a bullet traveling at 2000fps and experiencing a 20mph crosswind, the yaw angle would be the arctangent of the crosswind/velocity or 0.8 degrees. When the wind gusts, that yaw angle changes a bit to continue to nose into the resultant wind from the combination of wind vector and velocity vector.

Because the bullet weathervanes, only the frontal area and frontal drag profile matter. Those are described by the BC. Sectional density captures the mass as it relates to that frontal area and the form factor captures the drag as it relates to that frontal area. The result, along with velocity gives you total drag acceleration normalized for bullet mass. In effect mass is cancelled out of the equation because of how the trajectory is modeled. Does bullet mass matter? Yes, but the way and the degree to which is matters for all external ballistics calculations, all, is captured in the BC.

 

[wanderinwalker]

It's really, really simple, really. Two bullets of the same BC, launched at the same speed, have the SAME downrange drop and wind dift values. Bullet weight and diameter has NOTHING to do with it.

Don't overthink and overanalyze it. Get out and shoot some stuff at long range, beyond 300 yards and begin to understand what happens in flight. And if you're shooting long range paper targets and are concerned about wind, get yourself a nice hot rod 6.5mm and toss 142gr bullets.

 

[helotaxi]

When wind is calculated into that program, it assumes a steady wind from a constant direction. You won't often find that in the real world. Any change in wind or gust is going to buffet the bullet in flight. A heavier bullet will be affected less. It's simple physics.

Ever seen a large semi get blown off the road by a strong wind gust? What about a relatively lightweight sports car? You're right it is physics, and it's pretty simple when you look at the correct variables. Weight isn't one that you look at by itself here. Just like the semi, weight alone doesn't overcome wind. You have to look at sectional density because that is weight as it actually applies here, which is in proportion to area which is what determines the drag force.

You are correct about bullets turning into the wind. However, that only takes into account a steady state wind condition. It does not take into account varying wind and gusts throughout the bullet flight. Even though ballistic coefficient takes into affect drag acceleration, it does not take into account center of gravity and moment of inertia. Both of which will determine how fast a bullet can "turn into the wind". The mass and shape of the bullet are the only things that determine CG and moment of inertia. Therefore, they will in some way determine how much the bullet is affected during the time that it faces lateral forces before it can turn into the wind.

With a long range bullet design, the CoG and the CoP are not conicident with the CoP being forward of the CoG. The result is a shape that is unstable unless spun. It's actually the spin that both allows stability and forces the nose into the wind aligning the CoG and CoP on the same axis and keeping the forces acting on them from causing the bullet to tumble. Bullet wieght doesn't really play into this at all. Wind transients are not digital transitions, they are analog. The bullet doesn't have to instantaneously snap from 0.2 to 1.0 degree of yaw, there is a gradient. We're also talking about miniscule amounts of change. The overall result has been shown to be negligible both in theory and in practice.

Again, ballistics programs are a very good estimate. However, there are numerous reasons why they are not completely accurate in the real world. When you leave the realms of simplified trajectory programs and enter the atmosphere, bullet weight will have an impact, albeit a small one.

Modern ballistics programs, even ones that you can run on your smartphone are far from simple. The good ones actually allow you to program in wind variability along the bullet's flight path. In the case of those using the G7 BC, velocity effect on drag is accounted for as well. The simplicity or lack thereof of what you and I have access to, however, does not factor in to what actually has been measured and shown to be the case by people with a very advanced understanding of the forces involved, what matters and what doesn't and what the overall results are. This includes the DoD with access to radar sophisticated enough to track a projectile as it travels downrange to engineers with access to supersonic wind tunnels and the ability to program 6 degree of freedom simulations including all the variables and then test the simulations against real world results. Their findings are that mass, in an of itself, isn't important when determining how a bullet flies. BC tells you all that you need to know. The models also account for the variability of drag with velocity though the G7 model does a better job of this on average with a single number than the G1.

 

[murf]

wanderinwalker,

bullet weight and diameter have everything to do with this conversation. bc is equal to the sectional density of the bullet divided by its form factor. sectional density is equal to the MASS (weight) of the bullet divided by its DIAMETER squared.

heliotax,

your weathervane equation does not include bc, so yaw angle would have nothing to do with the shape of the nose of the bullet. it has everything to do with force applied in two different directions. if the crosswind was blowing at 2000 fps, the bullet would yaw at a 45 degree angle, regardless of nose shape.

murf

 

[murf]

cal30 sniper,

surface area is a square function. bc is a square function (diameter squared). force applied by the wind is a square function (mass times acceleration). i think surface area and bc are directly proportional in this case.

but, i'm no scientist.

murf

 

[wanderinwalker]

wanderinwalker,

bullet weight and diameter have everything to do with this conversation. bc is equal to the sectional density of the bullet divided by its form factor. sectional density is equal to the MASS (weight) of the bullet divided by its DIAMETER squared.

Which I am aware of, thank you, actually. But once you have BC (let's say .5), two bullets of .5 BC will fly the same at the same velocity. Doesn't matter if it's a 123gr 6.5mm, 107gr 6mm or 175gr .308", they will have the same drop and drift when launched at 2700-fps.

Going to a bigger bullet just for "more mass" thinking it will improve your wind resistance doesn't work. Say going to a standard 180gr .308" soft point hunting bullet versus a 155gr Palma bullet. The 155 is going to fly better (and has the advantange of being able to be pushed faster).

I was pretty sure after reading the OP, I was in agreement with his statement here:

Projectile weight doesn't determine how well a bullet performs at long range...it is the ballistic coefficient (BC) of the bullet.

I realize that weight and bullet length and caliber/diameter play into BC, but in the end it is the BC that determines a bullet's ballistic performance.

This of course is ignoring terminal ballistics, which tends to favor bigger bullets, all else being equal.

At the end of the day, what counts most is actually tossing the bullets in question down the tube and seeing what you get down range. I will happily shoot my 80gr .223 600-yard ammo all day against guys running 168gr .308s and not feel any disadvantage. But a 142gr 6.5mm slug trumps both, in spite of being lighter than the .308" bullet (it's longer and more efficient aerodynamically). And FWIW, the 123gr 6.5mm match bullets have pretty good BCs too.

From what I have seen on the firing line at matches, the winning combination is the best bullet that can be launched with tolerable recoil. If "the best" was the biggest and baddest, everybody would be using .300 Magnums with 220gr Matchkings or .338s with 250gr. Yet that's not what I see on the line...

 

[d2wing]

Which is why Sniper use larger bullets for ranges in excess of 1000 meters. What is true at 600 meters is not true at 1600 meters for example. The greater the mass the greater the potential B.C. At smaller calibers you cannot have enough mass to have optimal overall ballistics including total energy to overcome drag at long range.

 

[murf]

wanderinwalker,

agreed. 50 bmg would rule.

the op stated a fact: bullets of equal bc will have the same trajectory and the same wind drift. i'm not arguing this fact.

apparently, he is having a hard time understanding "why". i'm just trying to explain things in laymans terms and analogies to try and help him.

if this is a case of "the blind leading the blind", hopefully, someone else will chime in.

murf

 

[helotaxi]

your weathervane equation does not include bc, so yaw angle would have nothing to do with the shape of the nose of the bullet. it has everything to do with force applied in two different directions. if the crosswind was blowing at 2000 fps, the bullet would yaw at a 45 degree angle, regardless of nose shape.

Correct. BC does not affect whether or not the bullet yaws into the wind, it does so the same amount regardless of the BC based solely on the relationship of the crosswind component to the bullet's velocity. I never claimed anything to the contrary. But the angle of yaw is not what determines wind drift, merely what explains it and explains why the BC is what matters in determining the degree of wind drift. Wind effects are a drag function just like velocity decay and the yawing of the bullet into the wind is what keeps the cumulative drag effects of wind and velocity acting inline with the bullet's longitudinal axis.

 

[wanderinwalker]

Ah, that must be the part missed. We are talking the same thing in reality. All I can suggest is maybe the best way to clarify it all is a visual demonstration. Go to the range where different combinations are being used and see what is happening. Sit behind a shooter and scope the bullet path through the mirage in a humid day.

Ironically I was thinking about the 50 BMG after my last post. It may be the king of shoulder fired long range arms but I've read how in WWII it lacked range for aerial combat vs the larger 20mm guns. I guess my point is "there is always something better."

 

[cal30_sniper]

Thanks for the lesson guys. I was still confusing ballistic coefficient and drag coefficient. It all makes sense now, and I completely agree that the heavier bullet and lighter bullet will fly exactly the same.

As far as the aerial guns, I believe the difference was mostly that the 20mm shell had an explosive projectile, while the .50 used was normal AP. The problem with 20mm shells was they took up so much space, so firing length was very short before they ran out of ammo. Cannon shells have actually been used as shoulder fired weapons before though. The communists used 20mm anti-tank rifles as "sniper rifles" during Korea. I always thought that was interesting.

 

[d2wing]

You also take into consideration surface to mass of the same shape and time vector of force.

 

[helotaxi]

What is true at 600 meters is not true at 1600 meters for example.

Not sure what you mean by this. Physics don't change with range.

The greater the mass the greater the potential B.C. At smaller calibers you cannot have enough mass to have optimal overall ballistics including total energy to overcome drag at long range.

Mass only matters as it relates to cross sectional area. You could have a very efficient small caliber bullet if you made it extremely long or of something extremely dense both which would up the sectional density. Where you would run into problems with such a design is propelling it at a high enough velocity to realize the advantage of the increase in BC. Mass alone doesn't mean anything. A 120gn .30 cal bullet has no more potential for a high BC than a 68gn .224cal bullet even though it weighs significantly more.

The interesting thing about looking at all this from an analytical perspective is that you can scale a bullet up exactly and the BC will increase because sectional density will increase. Weight increases with the cube of scale while frontal area is only a square function. As such a bullet of the exact same proportions 2x as large as another will have 2x the sectional density because it weighs 8x as much while the frontal area only increases by 4x. That explains why on average larger calibers can have higher BCs. The .30 cals are at a general handicap though because the form factors are all sub par, especially when compared to a good 6, 6.5, 7 or .338 bullet.

If you want to think about something interesting, consider a bullet specially made to be fired with a sabot from a larger caliber rifle. You could optimize the bullet shape and length to get a very high BC and provided a fast enough rate of twist or dense enough material (tungsten or DU come to mind) stability could be achieved. Adding the extra area at the base from the sabot overcomes the internal ballistics limitations on velocity of the projectile caliber. The result would be a bullet with a very high BC and very high velocity but less recoil than if the same were attempted with a "conventional" design, i.e. jacketed lead core, full-bore bullet. Using a .224 projectile in a .338 sabot for example, you would need a 125gn projectile in .224 to get the same SD as a .338 bullet of 285gn. Getting such a projectile to have a good form factor wouldn't be difficult and using a high density material would address the length/stability issue. You should then be able to fire such a bullet to either equal the 285gn bullet's trajectory with a >50% reduction in recoil or greatly exceed the 285gn performance with a smaller reduction in recoil. I know that there have been sabot rounds in the past, but they predominately used light bullets and were only looking for speed numbers. The exterior ballistics were quite poor on the whole.

 

[murf]

d2wing,

wind drift in one easy sentence! (well maybe not that easy) force over time is distance. thank you for that.

murf

 

[d2wing]

Helo that proves what works in theory doesn't work in practice because your theory is not complete. Scale matters. This is long proved in practice. Range matters because you have to take into consideration things like sonic transition, mass to surface and momentum and stability. All this has been long be figured out. That is why the military uses .338 and .50 cal for long range snipers. No one uses 2 ft long .22 cal. Bullets. Keep thinking though, someone always comes up with something new. Shooting is proof. Results mean more than any theory.
You do have a point as spent uranium has been used to add mass so it must work. But I don't think that is practical.

 

[Big JJ]

Are bullet BC’s calculated by the mfg’s with consideration to the length and surface area or are they calculated just using the frontal shape and the weight?
Has anyone considered that two bullets of identical grain weight and caliber can have the exact same shape but one will be longer than the other if one is made of copper and the other made of lead?
This gives the longer one (the copper) a greater surface area and allows it to stabilize in the air better.
However this bullet (the copper) will be affected more by outside forces than the shorter (the lead) bullet.
Do those two bullets have a different BC’s from the mfg’s or would they be the same.
If so this will affect all shooting solutions.
This only comes up because I shoot on both sides of the Cali Condor zone.

 

[sixgunner455]

d2wing - DU is practical, but only in the right gun. Shoulder fired rifles, I don't think so.

 

[helotaxi]

Helo that proves what works in theory doesn't work in practice because your theory is not complete. Scale matters. This is long proved in practice. Range matters because you have to take into consideration things like sonic transition, mass to surface and momentum and stability. All this has been long be figured out. That is why the military uses .338 and .50 cal for long range snipers. No one uses 2 ft long .22 cal. Bullets. Keep thinking though, someone always comes up with something new. Shooting is proof. Results mean more than any theory.
You do have a point as spent uranium has been used to add mass so it must work. But I don't think that is practical.

The mil used/uses the .50 because it's a NATO standard cartridge and was the largest that they could realistically cram into a shoulder fired rifle. The ballistics are excellent as is downrange energy. They were more interested in the energy portion since the .50 is considered an anti-material weapon. They developed the .338LM because the .50 was too heavy, recoiled too much and had way more energy than needed for anti-personnel work even at over 1.5 miles.

As far as the transonic transition, mass doesn't play as much a role as bullet shape does. Mass has nothing to do with stability. Mass to surface is sectional density which has been more than covered. 90gn .224 bullets of standard construction are available right now. Using different material to make a 125gn bullet isn't that cosmic and a tungsten bullet could be shorter than a 75gn Amax being that tungsten is 1.7 times more dense than lead.

The idea is that you've got a bullet with a G7 BC better than .356 that you can comfortably fire in the 3600fps range from a .338LM rifle without a muzzle brake. The resulting exterior ballistics should be impressive to say the least. Like half the drop of a .338LM bullet at 2000yds. An extra 700yds before the bullet enters the transonic region. It would have plenty of energy for anti-personnel work out well past a mile and will have enough sectional density that it would punch right through body armor at that range. Probably engine blocks as well.

 

[helotaxi]

Has anyone considered that two bullets of identical grain weight and caliber can have the exact same shape but one will be longer than the other if one is made of copper and the other made of lead?
This gives the longer one (the copper) a greater surface area and allows it to stabilize in the air better.
However this bullet (the copper) will be affected more by outside forces than the shorter (the lead) bullet.
Do those two bullets have a different BC’s from the mfg’s or would they be the same.

The longer bullet is not more stable in the air, it doesn't work that way. In fact, the opposite is pretty much true. Longer bullets require a higher rate of spin to stabilize and can be very temperamental with stability.

As far as the effect that making a bullet from solid copper has on BC...

There is a practical limit on bullet length (as a ratio to the bullet caliber) both because you can only reasonably go with so fast a rate of twist and you still have to get the bullet in the case with room for powder and in the magazine. The end result is that on the whole, monolithic copper bullets have low BC's because for a given length they have a low sectional density compared to a lead core bullet. The Barnes bullets on the whole have a pretty good form factor, but the sectional density is a serious limiting factor. Hornady and Nosler advertise the same BC for their monolithic bullets of a given weight and caliber as the Interbond and Accubond respectively, but I'm not sure if I believe it. I think that they're putting all the extra length in the bullet shank, leaving the ogive and boat tails the same as the lead core bullets. That should make for very close to the same BC, but exactly the same... The problem is that you run out of length before you get to the higher weights when using a monolithic bullet thus limiting the highest possible BC.