used to practice this with my model 12...from the hip
back whe I was a teenager, and dumb as a post. My brother almost shot his foot off when, in a hurry, he loaded my model 12, with his finger on the trigger.
There are a number of older guns which will do this, but no newer ones, and for a good reason. IF you have the coodrination to pump the action, you have enough to pull the trigger a fraction of a second later.
Don't mess with the insides of a new shotgun to try and achieve some mythical rate of fire, it won't work.