Ballistic Coefficient
For the average reloader, terms such as “Ballistic
Coefficient” are meaningless, seeing as though the round being loaded doesn’t
have to be accurate beyond a couple hundred yards. However, if you’re one to
reload for the long-distance shot, the ballistic coefficient (BC) of the bullet
you are using plays a huge role in how well your round performs.
The ballistic coefficient is essentially how well the round
travels through the air; it’s a measure of how streamlined the bullet is. To
figure the BC for a particular bullet, you simply take the ratio of the
bullet’s sectional density compared to it’s coefficient of form. An easy
illustration is how a race car slices through the air as it rounds the track;
it produces a drag behind it due to it’s aerodynamic construction.
The bullet’s weight and construction has just as much effect
on it’s ballistic coefficient as the shape. In fact, the amount of powder
placed behind a bullet will affect it’s BC, as well. For the long-distance
shot, BC is what determines trajectory and wind drift. As you change the shape
of your bullet or the amount of powder you place in the round, the ballistic
coefficient will change accordingly.
As far as the shape of the bullet. A spitzer, or pointed,
bullet is more efficient at flying through the air than a round nose or flat
point. Also, the shape of the back end of the bullet, the point against the
powder charge, can change the BC. A boat tail, or tapered, reduces drag while a
flat base isn’t as efficient through the air. Let’s look at a few examples.
The Hornady 100 grain, 6mm round nose has a ballistic
coefficient of .216, while the 100 grain, 6mm spire has a BC of .357 and the
100 grain, 6mm boat tail has a BC of .400. Notice that all three of these
bullets are the same weight and size, causing them all to have a sectional
density of .242. However, the more streamlined the shape is, the higher the BC,
meaning the better the round will perform at slicing through the air for the
long-distance shots.
Hornady published the trajectory figures for these three
bullets to how the BC affects the long shot. For the 100-grain round nose, the
trajectory was -46” at 500 yards, the 100-grain spire was -30.5” at 500 yards,
and the 100-grain boat tail spire point was -28.6” at 500 yards. All three bullets
were fired from the same 6mm rifle with a scope zeroed in at 300 yards. This
just goes to show how important the BC is when reloading for long shots.
You can see how big the difference is between the round nose
tip and the spitzer tip, making it unfeasible to use a round bullet for a
hunting round. However, the boat tail doesn’t make much of a difference on this
particular round. However, at such a long distance, every little bit helps.
When it comes to finding the BC of a particular bullet, most
manufacturers will publish this information, often found in their reloading
manuals. This is, however, just one of many factors that should be considered
when reloading a round for the long shot. As always, experimenting with
different bullets and powder charges is recommended, especially when tailoring
a round for a specific rifle.
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