I have a theory concerning Kirika's weapon of choice, but I would need more info concerning its history and origins.
Has the Beretta M1934 Commercial been widely used in WW2 by either side of the Europe front?
I know that high level Japanese army officers used a more advanced Beretta at the Pacific front, but I am not sure about Europe...
It is entirely possible that Altena didn't just give ANY gun to Kirika, since she made sure Kirika used it during her training as a child, her time with Mireille, AND when Kirika returned.
Why bother giving her the same weapon?
If Kirika is just anyone, she can obviously kill easier using the same gun, since she used it so often it's like a part of herself...
But as Noir, she is not a specialist, but can kill people using any weapon with equal ease.
So why? You don't see Chloe retrieving every dagger she threw did you?
Just maybe... Altena obtained the gun a long time ago... In the WW2 battle-field of Europe... perhaps it was still attached by cold hard fingers to a severed limb of an air-raid/Blitz/artillery bombardment/land mine victim... perhaps it was the weapon that belonged to her family... perhaps it was the weapon that killed her family... Who knows?
Excuse the violent images... But that's what Altena has gone through in her youth, and no one can blame her for who she had became...