I used to make exactly these commercially ten years ago. My FEO advised me not to, so I stopped.
My ignore list: <Hidden information>
I have received enough springs through the post which could be gun parts but until assembly they are springs
Pete
Tbh if it's going Mainland theres little chance it'd be checked anyway, not that your average Post office worker would reconise it so you should be fine even if you post it built.
You'll Shoot your eye out Kid