Fortunately, or unfortunately—however you look at it—yes they are.
You may think this violates your Second Amendment right to “bear arms.” But the Second Amendment only bars the government from limiting your right to own weapons, not private citizens or businesses. A citizen or business has every right to decide whether or not they will allow firearms on their property—one of the cons of renting.
The second most common retort is that limiting gun ownership is discrimination against gun owners. However, unlike members of a religion, race, gender, ethnicity, and etc, gun owners are not a protected group. However, if a landlord were to target members of a specific race and prohibit them from keeping guns on the property—that would definitely be classified as discrimination. It must be an all or nothing rule, as with any other stipulations in a lease.
If landlords have no problem with renters owning and keeping firearms in their rental properties they should encourage tenants to purchase renters insurance. Firearms and other valuables are often a target of theft.
If a landlord does choose to prohibit firearms on their rental properties, they must wait until a lease expires and include the ban in the new lease. At that time tenants can either chose to comply or find other housing.
However, landlords should be aware that enforcing a ban on firearms is extremely difficult. If you think it’s tricky to catch a tenant with an unwelcome pet, it’s ten times harder to catch a tenant with firearms. If you do have proof of a violation, you can take steps towards an eviction.
Tenants should be sure to read the entire lease to make sure there isn’t a ban on firearms. If you’re a gun enthusiast at all, such a housing situation wouldn’t work for you. Additionally landlords should also double check their state’s law concerning prohibiting firearms. For example Minnesota actually has a law prohibiting landlords from banning tenants from having firearms.