My lease has a strict zero-tolerance crime policy that include no drugs as defined by section 102 of the Federal Controlled Substance Act. Although medical marijuana is legal in Florida, it's not at the Federal level and anyone who is legally using medical marijuana here is not allowed to buy a gun from a licensed gun dealer (this was a news headline recently after the Federal background check form was changed by the current administration).
My lease also requires my landlord to give a 24-hour notice of entry unless there is an emergency. I'm required to notify the landlord if anything looks amiss (such as moisture, mold, or bedbugs).
The brainteaser question is what happens if the landlord legitimately enters the premises because of a maintenance emergency (or otherwise valid reason) and happens to notice something that violates the lease?
Common sense tells me the tenant will likely be held in material breach of the lease and dealt with based on the terms of the lease (charged with the cleanup, eviction, possibly turned over to the authorities, whatever).
FYI -- I don't see any clause in my lease prohibiting a firearm. But there are quality-of-life items such as not annoying, endangering, or harassing other residents. The presence of a controlled substance is hard evidence, while these other items are not so clearly defined. A responsible gun owner will probably never let others know about the gun in the first place. Who wants to risk being burglarized by someone who knows there is a gun in the unit?