.308 trajectory comparison leaves me with a questions

[Guest BillOfRightsFan]

OK now I am confused. How can less power produces a flatter trajectory for the same bullet weight? Or am I making an assumption here...

I just got new ammunition. I am getting involved in long range shooting sports so I got 40 rounds of American Eagle 150gr .308 and 20 rounds of match grade Federal Premium at 175gr.

So here are the details:

American Eagle .308 150 grain. Zeroed in at 100 yards, drops 13.5" out at 300 yards.

Federal Premium .308 175 grain. Zeroed in at 100 yards drops 15.8" out at 300 years.

What? Shouldn't the higher powered round get to 300 yards faster and have less time for gravity to pull it down?

The box shows the power curve of the round and both boxes show something really interesting. The cheap stuff starts off with a much higher velocity and energy (2820 fps/2650 ft-lbs) but this degrades to 2180 / 1585 at 300 yards. The good stuff starts at 2600 / 2625 but at 300 yards is 2090 / 1705 - so lower velocity but much higher energy.

Curious. How does that work? I need the power of science to explain this!

[Dolomite_supafly]

OK now I am confused. How can less power produces a flatter trajectory for the same bullet weight? Or am I making an assumption here...

I just got new ammunition. I am getting involved in long range shooting sports so I got 40 rounds of American Eagle 150gr .308 and 20 rounds of match grade Federal Premium at 175gr.

So here are the details:

American Eagle .308 150 grain. Zeroed in at 100 yards, drops 13.5" out at 300 yards.

Federal Premium .308 175 grain. Zeroed in at 100 yards drops 15.8" out at 300 years.

What? Shouldn't the higher powered round get to 300 yards faster and have less time for gravity to pull it down?

The box shows the power curve of the round and both boxes show something really interesting. The cheap stuff starts off with a much higher velocity and energy (2820 fps/2650 ft-lbs) but this degrades to 2180 / 1585 at 300 yards. The good stuff starts at 2600 / 2625 but at 300 yards is 2090 / 1705 - so lower velocity but much higher energy.

Curious. How does that work? I need the power of science to explain this!

Have you actually chronographed the loads? Factory numbers are never correct from my observations.

Energy levels are not just velocity but also bullet weight.

I'll add more later.

Dolomite

[Guest 6.8 AR]

I'm guessing the ballistic coefficient has a lot to do with it. If I can say this right, the higher the

coefficient of two equal weight bullets will travel farther and faster, with all other variables being

equal. You have to make them apples to compare and just adding powder or reducing weight

isn't all there is. Boattails travel better through the air than boat anchors.

There's an Iphone app called "Ballistic" and several folks around here have the software that

does all that on PC/Macs. Take a Barnes 150 gr. TSX and compare it to the same bullet with a tip

on it and it changes the coefficient from .37 to .42 which will do better in it's flight.

Edited May 23, 2012 by 6.8 AR

[peejman]

Based on this...

The cheap stuff starts off with a much higher velocity and energy (2820 fps/2650 ft-lbs) but this degrades to 2180 / 1585 at 300 yards. The good stuff starts at 2600 / 2625 but at 300 yards is 2090 / 1705 - so lower velocity but much higher energy.

The muzzle energies differ by less than 1%. The "cheap stuff" retains 60% of it's muzzle energy at 300 yds. The "good stuff" retains 65% of it's muzzle energy at 300 yds. At 300 yds, the "good stuff" has 7.5% more energy. That tells me the "good stuff" has a better designed bullet. You also have to consider the aerodynamics of the bullet itself (referred to as the ballistic coefficient). Some bullets have very low drag and hence retain their velocity further down range. http://en.wikipedia.org/wiki/Ballistic_coefficient

[Morpheus]

The difference really shows up at longer distances. All else being equal, a heavier, slightly-slower bullet with proper design retains its velocity better than a lighter, slightly faster bullet, and this results in less wind drift. The bullet drop itself is relatively meaningless for known-distance shooting since it is PREDICTABLE and can easily be accounted for every time with the same basic amount of elevation adjustment. The advantage comes in at longer distances when the design of the bullet is such that it resists the influence of wind better, the total time of flight is less to the target way out there, and the exposure to UNPREDICTABLE wind condition is reduced.

These are just the basics. If you really want a full treatment on this subject, I recommend Bryan Litz's book.

[Tempest455]

I'm no expert by any means, just sharing what I've personally tried and also read up on. Seems 168g is ideal out to 600 yards. My son and I have had REALLY good results using 168g out to 400 yards. The 168g has been way more accurate compared to using 147-150g at 400 yards. Everything I have read and people I have shot with concured with that. However, they say past 600 yards, 175g is what is needed. Bought some 175g and hopefully will get to try that soon.

To date I have been using Black Hills which appears to be amazing off the shelf ammo. Going to try some Hornaday Amax soon and compare.

[Caster]

Ballistics gets deep man. As in Physics.

Think Momentum. Momentum is equal to mass times velocity. P = mv Bullet weight x velocity.

It gets hairy because an object set in motion will stay in motion until acted upon by an outside force. SO, you have gravity and the friction from wind resistance which means velocity isn't a constant [obviously]. Mass is constant though, and if you solve for P, a heavier mass will give you greater momentum over a a given range of velocities. Clearly there's a limit to this.

Think of it like a truck tire. In theory, the greater the diameter the slower the engine will run and thereby the less fuel consumed. This is true until the mass of the tire + the drag created by it overcome the peak efficiency of the engine. A really strong truck that drinks gas anyway [say a Ram Hemi or a 5.7L Tundra for example] would benefit from a larger tire MORE than a 4 cylinder Nissan or an S-10.

Like Tempest was saying about a 168g being optimum, the mass to potential velocity is about right. A 30-378 or a 7,82 Warbird on the other hand have much more to offer in the push department and can utilize a heavier bullet.

My grasp on the whole thing ain't so great, so I may be way off.

[Guest BungieCord]

All bullets begin falling to earth at 32 feet per second per second the instant they leave the muzzle. At a given range, any difference in drop is evidence of a difference in time of flight. Less bullet drop means lower time of flight. In this case, the faster of the two loads obviously has the lower time of flight to 300 yards, as you would expect, and as evidenced by it exhibiting less drop.

Relative power has nothing to do with it, it's all driven by time of flight.

[Caster]

That wouldn't make accuracy any better. A shorter time of flight would mean nothing if the lighter bullet [which would be more vulnerable to external forces] is thrown off course. A 50g bullet from a .220 swift would have a shorter flight time than a 750g Amax from a .50BMG but I wouldn't pitt them against each other at extreme distances.

Edited May 24, 2012 by Caster

[JeffsSig]

I think something has been overlooked here?

The Op wrote:

So here are the details:

American Eagle .308 150 grain. Zeroed in at 100 yards, drops 13.5" out at 300 yards.

Federal Premium .308 175 grain. Zeroed in at 100 yards drops 15.8" out at 300 years.

What? Shouldn't the higher powered round get to 300 yards faster and have less time for gravity to pull it down?

The 175 gr is not so much higher powered just a heaver bullet so the heavier one should drop more at a given distance?

PLUS

The extra velocity at a given range would be due to how the bullets spins along with its weight.

Look at throwing a football . One can throw a football for 10 yds with a lot of power even if its thrown incorrect with ball warbling.

But you can get about the same if not more power when the ball is thrown in a tight spiral at a longer distance.

And yes I may be way off here but it is somewhat clear in my weak mind.

[Guest 6.8 AR]

All of it plays in the game:drag,force and weight. It's not accurate to say all bullets will fall at the rate of fall

due to gravity. That is a free falling object that does that. The shape of the bullet, with all other things constant,

will determine the flight path. The same gravitational force is exerted on all objects. I think that is what he meant.

Get the highest coefficient bullet for the weight you want to shoot, and get someone else to help you with the

powder for your accuracy to get up there. They won't necessarily come from a box of Federal's, but that doesn't

say anything negative about Federal's, either.

A good physics class will help you a bit, also.

[Morpheus]

...or just read this book (per my first post): http://www.appliedba..._files/Book.htm

It is the new standard reference for this particular subject.

For long-range known-distance shooting, the key factors are how long the bullet is subjected to the unpredictable wind influence (time of flight), and how well the shape of the bullet is able to resist the unpredictable wind influence (ballistic coefficient). Again, for known distance shooting, the elevation adjustment is relatively unimportant since it is much more predictable with altitude, temperature, and barometric pressure inputs producing a predictable result. The difference between a good long-range known-distance shooter and a great one is their ability to observe the unpredictable wind influence and determine what to do about it.

Come out to our 1,000-yard match this Sunday at AEDC in Tullahoma and see these factors put to the test by some of the state's top shooters.

Edited May 24, 2012 by Morpheus

[peejman]

All bullets begin falling to earth at 32 feet per second per second the instant they leave the muzzle. At a given range, any difference in drop is evidence of a difference in time of flight. Less bullet drop means lower time of flight. In this case, the faster of the two loads obviously has the lower time of flight to 300 yards, as you would expect, and as evidenced by it exhibiting less drop.

Relative power has nothing to do with it, it's all driven by time of flight.

... and aerodynamics. The above is true when both are fired from a level barrel. As most rifles are "zero'd" at a range beyond 1" from the muzzle, the angle of the barrel and the trajectory of the bullet factor in. The combined effect of both velocity, mass, and aerodynamics are directly proportional to the distance traveled. Differences become greater further downrange.

Edited May 24, 2012 by peejman

[nightrunner]

To the OP, the measurement of grains (150 or 170) is the BULLET WEIGHT not the amount of powder in the case. They likely have close to the same amount of powder but trying to push the lighter bullet it goes faster and the heavier bullet goes slower. That is assuming exact same bullet construction, sectional density, ballistic coefficient, etc.

[Jonnin]

I think something has been overlooked here?

The Op wrote:

So here are the details:

American Eagle .308 150 grain. Zeroed in at 100 yards, drops 13.5" out at 300 yards.

Federal Premium .308 175 grain. Zeroed in at 100 yards drops 15.8" out at 300 years.

What? Shouldn't the higher powered round get to 300 yards faster and have less time for gravity to pull it down?

The 175 gr is not so much higher powered just a heaver bullet so the heavier one should drop more at a given distance?

PLUS\

Negative. All objects drop at exactly the same rate. Now, air resistance falling can come into play (a feather, for example, will not hit the ground at the same time as a bullet if you drop both) ---- but if you took away the air, and dropped them on the moon, they would both fall the same!

if you shot the rifle at exactly 90 degrees (exactly parallel to the ground in a perfectly flat area) and dropped a bullet from the muzzle height at the same time, they would hit the ground at the same time (assuming there is no outside factor to raise the bullet from the rifle).

That is why velocity is king. You have however many seconds... if you fire from a bench rest parallel to the earth, you are what, 4 feet off the ground? The bullet has 4 = 1/2 32T^2 solved for t (time) = 1/2 a second. If the bullet is booking it at 3k FPS, then, it hits the ground in 1500 feet assuming no loss of velocty (and there will be some). Of course, no one shoots parallel to the ground, you shoot up a little, and zero the scope to account for that, etc. Angled up just 5 degrees gives you a lot more hangtime due to that large initial velocity upwards. 5 degrees up at 3000 fps is 261 fps upwards, in this example, which is significant!

[Guest Papabear]

Jonnin has a point that velocity is king. In long range shooting you try to keep the bullet supersonic for as long as you can. The bullet's trip back through the sound barrer will destabilize it some. This is why calibers like the .416 and .406 have such good long range accuracy. They were designed with slippery bullet profiles for low drag, heavy bullet weights to minimize outside forces such as crosswinds on the flight path and to stay supersonic for as long a distance as possable. But they are also rounds that were designed just for long distance shooting. Most calibers that the majority of people use were not designed for just long range like the .308. So thats why most long range shooters handload so they can pick the best profile bullets and best powder burn rates to get the velocity up for as long as possable.

[Guest BungieCord]

... and aerodynamics. The above is true when both are fired from a level barrel. As most rifles are "zero'd" at a range beyond 1" from the muzzle, the angle of the barrel and the trajectory of the bullet factor in. The combined effect of both velocity, mass, and aerodynamics are directly proportional to the distance traveled. Differences become greater further downrange.

It's irrelevant whether the barrel is level, or at what range it was zeroed. Unless he re-zeroed when switching bullets, and if he used the same point of aim, then the angle of launch in both cases would have been identical. Regardless of the range, regardless of the trajectory, regardless of bullet mass or BC, if the angle of launch is the same, at any given range, the bullet with the shorter time of flight will always have the higher point of impact.

Always.

[peejman]

It's irrelevant whether the barrel is level, or at what range it was zeroed. Unless he re-zeroed when switching bullets, and if he used the same point of aim, then the angle of launch in both cases would have been identical. Regardless of the range, regardless of the trajectory, regardless of bullet mass or BC, if the angle of launch is the same, at any given range, the bullet with the shorter time of flight will always have the higher point of impact.

Always.

Agreed. But the launch angle when zeroed would necessarily be different because of the different trajectory of the bullets. I assume a long range shooter would have recorded zero setting for different loads.

[atlas3025]

To the OP, the measurement of grains (150 or 170) is the BULLET WEIGHT not the amount of powder in the case. They likely have close to the same amount of powder but trying to push the lighter bullet it goes faster and the heavier bullet goes slower. That is assuming exact same bullet construction, sectional density, ballistic coefficient, etc.

I think this nails it. I believe the problem is that the OP is assuming the 175 grain round is "more powerful" (ie faster). Typically, in any given caliber, the lighter load (smaller gr) will be faster and will not drop as much. As has been said, sometimes the heavier bullet will start slower, but finish faster...meaning when it leaves the barrel is not going faster, but when it hits the target 500 yards away, it is...thinking of 6.5 Grendel vs. a 6.8 SPC. Sometimes the heavier bullet (maybe all the time) carries its speed further because it has a longer and leaner shape.

Think of a drag race. The bullet that gets there quicker will drop less...though it may not necessarily be going faster when it hits the target.