Most of the DQable offenses are not something you can usually warn someone about. If a guy is doing a reload with his finger on the trigger and you see it, it's already a DQ, there is no warning to give. Most of the warnings I've given involve silly stuff happening during make ready (almost exclusively with hammer fired guns) or after unload and show clear, or in between stages where I try to point something out to a guy if he is oblivious to what he's doing (like doing reloads right on the 179 degree line every time, or something like that)
If you're doing that 179 degree reload during the stage, yelling something at the shooter to distract him is only going to make it worse