If a gun is on the mantle in the first act, it must go off by the third. Otherwise leave it out.
If a gun is on the mantle in the first act, and you want it to go off with deadly consequences, let it be a loaded gun that’s not locked up or anything.
If a gun is on the mantle because a specific character put it there, write that character as a roiling emotional sea of strident politics, antique traditions, and fragile masculinity.
If a gun is on the mantle or table or shelf at a gun show, any character of any moral quality can buy that gun, no questions asked.
If a gun is on the mantle because a character 3D printed it themselves, audiences expect that character to be the kind of repellent creep that even a gun show vendor turns down.
If a gun is on the mantle for easy access by a member of a well-regulated militia, your story is set in the Colonial era.
If a gun is on the mantle in the first act, and it comes down from the mantle after all the characters started boozing, wow, you sure do hate your characters.
If a gun is on the mantle in the first act, and it kills a character, that character can never return to your world.
If a gun is on the mantle and you create a Senator character or Congressperson character, they will be powerless to affect that gun.
Internet posts about gun control have as much real world impact as a fictional gun on a mantle.