If you click LMB or RMB just in the right moment while your shotgun is reloading (in a moment when it won't shoot immediately but rather "schedule" a shot to be made after the animation has finished) and phys-swap before the shell load animation is done, when you swap back to your shotty, it will make a shot
If you click LMB or RMB just in the right moment while your shotgun is reloading (in a moment when it won't shoot immediately but rather "schedule" a shot to be made after the animation has finished) and phys-swap before the shell load animation is done, when you swap back to your shotty, it will make a shot