I've got very limited experience with FLO's and can only comment on how things are done round here, but it's not a case of the 3rd degree at all.

I fell for these 'interrogation' stories and was crapping myself to be honest, but it didn't work out like that at all in practise.

Tell him the absolute truth is the best advice I can give you, that's what I was advised by people who know the score.
If you don't know the maximum danger range of a rimfire round or a shotgun cartridge then admit you don't - he'll tell you what it is. The interview isn't an exam.

Good luck