Most master bedrooms are at or near the end of a hallway and have very little "common wall" with hallway space. They may have common wall with other bedrooms but that's not the point here.
If you have a little space, put a knick-knack cabinet or narrow bookshelf in front of the wall (in the hallway). If you happened to fasten a 1/8" steel panel to the back of the unit, so much the better.
Protection only needs to be about armpit high at the door frame. This will allow a kneeling position while covering most of your body. Some kind of pedestal stand for an oak or cedar chest full of blankets or bedcovers would do fine, if a little short on height.
Just as important (to me anyhow) is to make sure that
walls of the guest and children's bedrooms are protected from
your line of fire. This means adjusting the layout of the kid's bedrooms so they aren't sleeping along the hallway wall or have the head of the bed close to that wall.
If you install the solid core door, also consider some "dead throw" bolts. These are usually used in double-wide doors to keep the non-functioning side of the door closed. Some use a metal loop at the bottom or top to retain the door.
Install the mounting piece with 2" screws. If installed vertically with a hole into the flooring, be sure to install a tight-fitting metal collar at the floor level. If mounted horizontally, one top and one bottom, be sure to use a 2.5" or longer screw to mount the locking loop end.