From easyjet:
Can I take my sports firearms on board?
For an additional non-refundable fee we can take sporting or competition firearms and a maximum of five kilos of ammunition on board.
Please note that firearms must be packed individually in a suitable gun bag and ammunition must be packed securely in a hard wooden or metal box. Ammunition with explosive or incendiary projectiles is prohibited.
In addition, you must inform us in advance that you wish to take firearms or ammunition on board and you will need to show valid documentation of proof of ownership and legal authority for the possession of the firearm at check-in (even if you are carrying only ammunition). Please make sure you arrive at check-in at least 90 minutes prior to the scheduled departure of your flight in order that we can carry out all the necessary checks. If you arrive after this time you may not be allowed to fly.
Please call our Customer Experience Team on 0871 244 2366 to let us know if you are travelling with a firearm or ammunition.
For more information about sports/competition firearms and baggage allowances check out our Carrier's Regulations.
Not sure what difference if any it will make if the firearm is not assembled, but I think a call to the airline would be a good place to start.