Did you use regular Royal mail?
I'm a wee bit confused because when there's tax to be paid it's usually cash on delivery or you pay it when you pick them up at the post office.
It's a flatrate 25% tax for any goods entering the country with a value higher than 200NOK\£17 unless it's a gift where the highest possible value is 1000NOK\£80.
However, if you're just going to have these guns in the country for said event you should contact the customs office and tell them just that, according to our laws they should drop any tax claims at that point..
(IE bringing the guns back to the UK when you leave = no tax)
As for not knowing that they had to go through customs won't resolve the situation, it's in the disclaimer..
If you have any further questions about this feel free to PM me beause I know pretty much everything there is to know about the evil customs' laws in norway.. :blush:
Sometimes I wish Norway was in the EU, none of this extra tax-bs annoying the hell out of me, VAT FTW