Biathlon Rules governing the Winter Olympics have varied and
are subject to the individual event in some cases.

In general:

This discipline was based on a military application and evolved
from warfare during wintertime in nations that have snow and
mountainous conditions, and therefore, the rifles used were
the currently issued service arms.

The modern sport uses a 22 long rifle cartridge chambered in a
bolt action, or similar, rifle that must be slung over the back
during the ski race and the winners are determined by the fastest
elapsed time covering the ski course.

During the race, the competitors must stop at shooting ranges for
both Prone and Standing fire at 50 meter distances. The targets
are constructed of steel; 45mm for Prone and 115mm for the
Standing position and when hit, they fall, thereby permitting an
instant method of scoring. (Note: 50mm is approx. 2 inches).

Obviously, this is a very strenuous "cross-country" ski sport
which increases the competitor's heart rate to nearly 200 beats
per minute. Dedicated entrants are able to reduce their pulse
rate well below 100 within seconds, in preparation for the task
of firing at the difficult targets.

As one would expect, there are targets that are missed. For that,
there is a penalty which varies for the individual races. Some
events impose an additional 150 meters of skiing for each missed
target or a one minute time increment added to the total race time.