Dispelling Myths about Trajectory


12/14/2005

Trajectory and Ballistics are both fascinating and confusing.

In the past couple of months I've heard some pretty wild misconceptions about bullet paths.

It's pretty simple really. When the bullet leaves the barrel it starts slowing down and dropping immediately.

There are no mysterious acrobatics involved.
No super natural powers that make the bullet climb.
No secondary propulsion to accelerate the bullet.

Basketball might be a good analogy. The shooter looks at point "B" and shoots from point "A". The shooter knows the REQUIRED ARC the ball must take to get to point "B". The farther the shooter is from the goal the higher the arc. Put two players side by side aiming at the same goal, their arcs will not match.
" BALL - ISTICS "

Firing a bullet is no different. We do not calculate the required arc but it has to be there.

Look at any bullet chart and the bullet seems to start at point "A" travels above the level of the muzzle and comes back down to point "B". What the charts do not show is the fact that the chamber is BELOW the muzzle.

Of course there are many factors that determine the point of impact or point of "Zero" and that is where ballistics come into play.

Look at the following table.

To be "dead on" at 150 yards, you can see the required arc. It's not an option, it's not a coincidence, IT IS REQUIRED.

Cartridge
Bullet
Muzzle Velocity
Required arc at 100 yards
Zero at 2 points of the arc
30-30 150 2390 1.5 Inches High 27 yards and 150 yards
300 Win Mag 150 3100 .55 Inches High 49 yards and 150 yards
220 Swift 50 3870 .21 Inches High 70 yards and 150 yards

You can easily see that a faster bullet has less arc. Hence the term "Flat Shooting Rifle" or "Flat Shooting Cartridge"

Because you do not sight down the bore, and we can only see in a straight line the bullet must exit the muzzle below zero or point of aim. How much below zero is in exact proportion to the sight plane you are using above the bore. For example: High scope rings and a 50mm objective lens might put your actual sight plane 1.5 inches above the bore. Consequently, the bullet starts 1.5 inches below zero.

It stands to reason then that you sight at zero and your bullet actually crosses your sight plane in it's arc in route to the target. Put another way, the bore of your scope is not parallel to the bore of your rifle. Instead, the sight plane of your scope is level while the line of the bore is on an incline.

While slight, the taper is visible and you get the idea that over distance the two lines (the sight plane and bore line) would cross.

So what about the "rule of thumb" "if it's on at 25 yards then it will be back on at 100"?

We've all heard it and Now We Know. Since the bullet starts below zero, and the bore line is an incline, the bullet might actually cross zero or "be dead on" at 25 yards and then back on at 100 yards. The odds are not good though.

Cartridge
Bullet
Muzzle Velocity
25 yard Zero
Where is it at 100 yards ?
30-30 150 2390 1.89 inches High
300 Win Mag 150 3100 3.04 inches High
220 Swift 50 3870 3.53 inches High

While sighting in at 25 yards is easy to accomplish, don't count on being back on at 100 yards. And "slightly off" at 25 yards can be feet at 100 yards.

These numbers were all crunched with help from Load From A Disk Software.

Bill Wade


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