In the UK firearm regulations are very strict, but I would like to learn about them like in the other countries. Is it okay to print 3D gun designs if I make sure none of the parts can shoot real bullets by using weak plastic that would explode in the hand if fired with a real bullet?
Is it legal in the UK to 3D print a firearm design if it is made impossible to fire?
Maybe, maybe not
If the result is merely a toy, trinket etc then there is no offence per se. However it will be unlawful if it meets the definition at s.57 of the Firearms Act 1968 of a firearm, or used as an "imitation firearm" to commit any number of offences in the Act:
(1) "firearm” means-
(a) a lethal barrelled weapon
(c) a relevant component part in relation to a lethal barrelled weapon
3(1B) In subsection (1)(a), “lethal barrelled weapon” means a barrelled weapon of any description from which a shot, bullet or other missile, with kinetic energy of more than one joule at the muzzle of the weapon, can be discharged.
(4)(b)...“imitation firearm” means any thing which has the appearance of being a firearm ... whether or not it is capable of discharging any shot, bullet or other missile
In Germany, there are similar prohibitions against building a firearm or significant parts of a firearm, and also against gadgets which give the appearance of a firearm.
I expect that it would be possible 3D print something which meets neither definition, but doing so is tricky for an amateur. When a German IT publisher printed a gun some years ago, they played it safe and let a licensed gunsmith do it.