As an add on to existing answers consider your question
Can I bring a single shot of live ammo onto the plane from US to UK as a souvenir?
as being seen by security authorities as looking something like
Can I bring a single shot informal firearm onto an aircraft if I also carry the single live ammunition round with me but separately from the firearm ?
Seen in this light, without knowing the specific regulations it's obvious (or should be :-) ) that the answer is probably "NO!!!", or at very best "Only after very large amounts of questions and paperwork, probably not as carry on, probably not without significant pre-application period and, probably, no!"
If you have Facebook access then this page will be informative Bootleg Gunz - The Bazaar of Bizarre Guns will be informative re what can be achieved.
Genuine: 22 LR, Australia
A "bootleg" / 'slam-fire' / ... firearm can be implemented in many ways from equipment and material that does not resemble a conventional firearm. The common key component to any such weapon is a live round - if you have that the weapon proper is doable.
Bearing in mind also that if you are able to carry such a round onto an aircraft then Bob & Carol & Ted & Alice (and Uncle Tom Cobbly and all) also can and a number of you can then pass your ammunition to someone who has a/the weapon.
This is live. Made in 1916.
This would not be permitted on a flight :-)
(Not me - that's a friend who hadn't thought it through. We lived. )