Of course an opposing pundit might add that it's not true for the victim.
Oh okay. The reason I made that my slogan is because my project is about advertising a weapon that is good for WW1, so I chose rifles.
Would that be okay?
Yes.
Yes, the grammar in your slogan "Rifles are a man's best friend" is correct. However, I would like to note that slogans are typically short and impactful, and this slogan may not be universally applicable or appropriate for all audiences. If you're open to suggestions, I can help you brainstorm alternative slogans that might better convey your project's message. Please provide more details about your project or any specific theme you have in mind.