I always "bore sight" a new scope because I usually can't wait to get out to the range and I want to fiddle with it while I'm still at home. Iset a cardboard box on my dining room table open side up.Then cut a couple of notches in two opposite top edges to set the rifle in. Put masking tape over it to secure it to the box if necessary - or it might just lay there OK by itself. Take out the breech plug and look through the bore. Raise or lower THE BOX with magazines, newspapers, or whatever to line up the center of the bore on some small object 50-75 yards away (I've even used the corner of a window on a neighbor's house as a "target"). If you cannot remove the breechplug and can still use the sights with the scope on, you can line up the sights on something. Then, without moving the rifle, move the crosshairs on the scope to the object you're sighted in on. You'll be very close when you fire that first shot.
Good Luck