find a mace and swap your rifle in your hands with it and swap again.
I have not reproduced it with a other mace at a other location, yet.
I was on an exp 1.person server.
I was doublecarrieng a blaze in my hands and a repeater on my back, but not a secondary melee on my back. Inventory was kind of full, so no place for the mace in the backpack either.