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You mean the hop-up rubber or the chamber?
For the rubber, replacing the hop-up rubber in a CYMA gun with a better one can give you a great increase in accuracy, in most case.
For the chamber, the general rule is don't replacing a working-fine hop-up chamber unless there is a very good reason. A small variation in dimension (mostly, the length) can give you a very weird result and it is kind of a pain to correct it if you are not familar with this type of issue.
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