Again, I beg your indulgence, because I'm really not trying to be vague, but it would really depend on the circumstances.
If a firearm part is seized by police, and that forms the subject matter of a criminal matter that then goes to court and proceeds to trial, etc., at the termination of that trial, the Crown has the opportunity to make an application to the court that all those things seized by police, which would remain in police custody throughout the duration of that matter, are then forfeited to the Crown. When they are forfeited, they become subject to a disposal regime.
In the circumstances that I'm now describing, that part has no opportunity to go back into circulation.
Again, I can't account for all possible circumstances, but there are regimes in the code that deal with the problem that I think you're raising.