The questionarres are sent out 99% of the time without the box being opened at all, (and usually by the couriers and not actually customs), its prompted by the declaration the sender put on it. On occasion x-ray scanning may prompt it.
The word 'gun' does not have to be avoided - guns are legal in the UK, and no matter how much people say 'marker' its still a gun in many peoples eyes. However avoiding the word gun will avoid the knee jerk reaction.
Does the questionairre lead its questions along the lines of firearm, replica firearm etc? These are to clarify if its a firearm requireing a licence, legally deactivated firearm, airsoft / BB gun that is a RIF and requires the skirmisher defence under the VCRA, another exemption or another defence.
Answer yes/no to the appropriate questions, and add in the final remarks that it is a paintball marker.
The courier decides if your answers are good enough for it to be satisfied that they have fulfilled their responsibilities
They 'declare' it to customs, customs decide if they have any interest
The box may or may not be opened to verify
Then it gets sent on to you - with or without an import VAT bill