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One could just carry a HK P7 M8 or M13...
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The firing pin & spring assembly 
mate with the firing pin bushing 
to form a self-contained "striker assemby", which is inserted into the rear of the slide.
When the "squeeze-cocker" in the grip is fully depressed, the firing pin (in the center, above) extends beyond the bushing, the weapon is cocked -- and will fire when the trigger is pulled.
BUT -- if the squeeze-cocker is depressed just until the firing pin is flush with the back of the bushing, a simple 90-degree turn of the assembly (with your thumb) "unlocks" the assembly, and further depressing of the squeeze-cocker makes the whole firing assembly pop right out into your hand.
At that point, the weapon is totally inert and non-fireable. (P7 owners need no stupid gun locks!)
With practice, removing the "striker assembly" (firing mechanism) and dropping it into your shirt pocket takes a whole four seconds.
Replacing the assembly (with the same one -- or another...) [thus returing the P7 to full functionality] takes about the same time and effort.
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Of course the HK P7s are no longer in production -- so the laws as written don't apply. (And the surviving firearms [and parts] are now hellaciously expensive!)
OTOH, I understand that Glocks use a similar "striker assembly"...
...but, I've ever tried to remove and replace one...