is the rap4 bb pellet airgun good?

I am considering getting the rap4 bb pellet airgun it has semi and full auto removeable carry handle and can have many attachments.
what is the best airsource for it (must fit in the stock)?
can you take off the handguard and put on a full ris rail with rails on all four sides?
how fast does it shoot?
what upgrades can it take?
what accesories should i put on it?
is it better to get a red dot or reflex sight?
what is the best kind off bb or pellet to use?

 

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