Almost all names of German-manufactured weapons and equipment do not use hyphens, including present day ones. Ranging from MP40, StG44, MG42, H&K MP5, you name it. I checked the Wikipedia page of this shotgun. I suggest aliasing it into shorter FP6 for the sake of simplicity. Feel free to change this BUR alias into the more "specific" H&K FP6, H&K Fabarm FP6 or whatever. It's up to your thoughts.
Even the Wikia page of Haruka from Blue Archive states that her weapon name is written sans hyphens.
Thank you for your time and suggestions.