In halo 3 there was a key combination where you could conceal your weapon or lower so you could not always run around with waffled weapons.
But how exactly does the key combination in halo 5 actually work?
How to lower the weapon in the game itself I said
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> How to lower the weapon in the game itself I said
Go into Custom Games, Game Mode Options, then to “General” and set “Machinima” to a ON.
Start the game and, with default controls, hold X and press down on the d-pad for 2 seconds.
I don’t think you can " lower weapons" in normal matchmaking for some reason.
Sorry if I’m misunderstanding 
You need to put machinima controls on in customs
then you hold you reload button and down on the dpad to lower your weapons