Tennis balls are hollow, pressurized balls of vulcanized rubber, with a fuzzy felt-like covering, approximately 2.6" in diameter. They are sold in many retail stores (including Wal*mart) for use in the game of tennis.
Somewhat smaller (and significantly cheaper) alternatives with the same basic construction exist, and are marketed as "fun balls" or pet toys.
The fuzz on the ball increases air resistance above what would be expected of a sphere, to a Cd of about .6 at high velocities. (See for example this research)
This, coupled with the low sectional density of approx. 10.5g/in2 exterior the launcher, cause the unmodified tennis ball to be a short range projectile; range is limited to about 150 yards (137 meters), even with a powerful launcher. It should be noted that this is not necessarily a bad thing.
The friction required to load a tennis ball in a 2.5" sch 40 barrel is about 12 pounds . If one finds this excessive, they generally correct the problem by burning the fuzz off.
Additionally, one may seek to increase the range or damage capabilities of the tennis ball. This generally entails weighting the tennis ball by injecting water, or inserting pennies or other small, dense objects. Optimal mass increase may be calculated with repeated trials using the GGDT's integral external ballistics calculator.
Use and target performance
Due to having to reduce the OD of the tennis ball from ~2.6" to 2.445", there is a significant amount of friction; 2.1 - 2.7 psi, average of 2.5 psi, for a force of about 12 pounds (54 newtons). This complicates loading, slightly reduces muzzle velocity, and during launch will scorch the fuzz on the ball.
The inexpensive "fun balls" are slightly smaller, and thus have reduced friction.
As an alternative, some use SDR 21 or 26 2.5", which have IDs of 2.581" and 2.635" respectively.
Diameter, unsqueezed: 2.6" (65mm)
Mass, unfilled: 57g
Friction; in 2.5" sch 40: 2.5 psi; 12 pounds (.18 k/cm^2; 54 newtons)