Question: What's the best tennis ball for a ball machine?
Answer: Pressureless balls make sense for a ball machine because they don't lose their bounce while sitting around waiting to be hit. Most ball machine balls wait a long time for one hit, so it is time, not use, that they must endure. Avoid oversize balls, which can jam some machines, and heavily worn balls, which won't shoot as far in machines that depend on air-tube pressure to propel them.
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