How do i make a gun where if the projectile hits the ground it places a block?

You need to create a procedure that executes the "/setblock" command, then select the procedure in your ranged item at "When projectile hits block" and "When projectile hits living entity".

But if you only want it to place a block when the bullet hits the ground, but that it doesn't place a block when hit an entity, then just select "When projectile hits block"

You don't need to use the /setblock command as there is a "place block at x y z" procedure block

ClOud1076 that procedure will just place the block at the player

nvm Im stupid