Pending "WHERE" the compressed air enters barrel ? ( Spring piston or PCP or SSP or Pumper )
Most PCP's will use a transfer port coming into the bore sideways a bit in from the breech end.
In those cases you NEED the skirt of a pellet or slug to be seated just past the edge of transfer hole so pressure bears 100% at the tail end of either type projectile. Most pellets have a skirt cavity requiring a LONGER seating probe than a typical flat of dish base slug which would use being a shorter probe for equal seating depth.
Generally incrementally seating PAST the transfer port increasing the space/volume that must come up to pressure for the launch will trend Less Efficient
Second part of your inquire ... FREEBORE / THROAT
Some will have no increase in Throat keeping rifling intact all the way to breech entry ( Mostly Pellet shooters )
Slugs due to full contact will LOAD EXCESSIVELY difficult is such a setup & generally use some freebore just At or just slightly smaller than slugs O.D. running it past the transfer port ( Based on Slugs length ) so when seated the nose of slug is just starting to indent into rifling.
There are variants one can apply and that comes with experience and testing in a case by case basis.
Good luck & have fun !!!
