Theres a section for that on the norwegion website rob i glanced over it andy seems to have got loadsa info pretty well organised on that front by looks of things copied from the website
Temporary import document
Current law states that there is no restriction on the temporary import of air rifles of cal 4.5mm (air rifles are classed as ”other goods for personal use” ) Furthermore, there is no current police form for the monitoring of the temporary import/export air rifles however, we hereby request competitors to fill out form GP 6144-E in two examples (example here). You should also present your World Championship invitation on entry and a copy of this email from the Customs information bureau.
Go through RED channel upon arrival airport/customs point. Customs should stamp both examples, retaining one copy. Retain the second copy which MUST be submitted to customs on departure from Norway – without fail !
Norwegian CUSTOMS (TOLL) and POLICE (POLITI) officials are very helpful and courteous – please return their civility.