Are Firearms Classed as "dangerous/Offensive goods" ?

[Dark Skies]

Your landlord, or indeed anybody, shouldn't be told what's in your safe.
That being the case why would your tenancy be in jeopardy?

I suspect the landlord's main concern regarding dangerous goods and such are things that are more likely to present a risk - storing extremely flammable and / or toxic fluids and materials. Real life risks rather than unloaded firearms safely locked away.
I wouldn't sweat it.