While velocity increases performance of rifle and handgun ammunition the maximum usable velocity for a shotgun shooting lead shot is around 1,400 fps with most ammunition running around 1,200 fps. Push a lead shot load out the end of the barrel much faster and you will "blow" the pattern as the wad is shoved through the shot column yielding a doughnut shaped pattern.
Steel shot being less dense can be pushed at a slightly higher velocity of around 1,500 fps but because of this lower density it looses velocity faster than lead.
Because of the size of the bore, the relative thinness of shotgun barrels and the velocity restriction most shot shells run around 8,000 to 9,000 psi in the 12 ga.
As the gauge gets smaller the pressure of the shells does go up with 20ga ammo running in the 10K to 11K psi. The velocity of the shot does remain the same though at around 1,200 fps being typical.
A typical 1oz rifled slug on the other hand will have a velocity of around 1,700 fps, and the sabot versions will be around 1,800 fps.
I have heard that .410 slugs are equal in kinetic energy to .44 Magnum bullets, which have about the same diameter, but operate at a much higher pressure. How is this possible?
Comparing the .410 1/4oz or 1/5oz slug to the .44 mag or .41 mag may yield similar kinetic energy numbers but remember that the barrel of a shotgun is a lot longer which yields velocities of around 1,700 fps for a .410 slug. The 3" .410 slug at 1/4 oz is only approximately 109 grains, about 1/2 the weight of a typical .44/.41 mag bullet of 240gr/210gr. The equivalent energy come from the way energy is calculated using the square of the velocity. When 1,700 fps is squared it is about 2 times the resultant number of 1,200 fps (the typical .44 mag velocity) squared. The projectile of the .410 being 1/2 the weight of the magnum pistol projectile makes the results come out pretty much equal. If .410 slug was shot from a Taurus Judge with pistol length barrel the resulting velocity would be much slower and the kinetic energy would also be a lot less than a .44/.41 mag pistol.