I had the similar experience flying to Italy.
I was carring a semi assembed AK gearbox in my backpack and was seized in UK by the security , a lot of questions, almost lost the plane and collected
at the return in UK.
I was carring a AEG inner barrel in my hand suitcase coming to UK. Again was seized at the security check and lost it.
On the other hand, when I went to Japan, I bought PTW M4 parts, a RIS and a Tokyo Marui revolver , fitted in the checked luggage and i did not have any problems.