How does one put a horizontal drain (ABS. 1 1/2") pipe in a wall that is already in place?
Get a bigger hammer (and use a wood block to protect the end of the pipe). Large pipe (3&quot;, or maybe even 2&quot;) would not slide into there but 1-1/2&quot; should.
Make sure the first hole particularly is drilled generously oversized so the pipe won't bind in the hole while you're forcing it in. It may even help to drill that first hole slightly off perpendicular, angled toward the surface of the wall in the direction the pipe will be inserted.
If you're trying to start the pipe going through that stack of three wall studs that could make life miserable. Insert the horizontal pipes from the right instead if that's an option, or from a couple studs back to the left. It's OK to drill holes in other studs so the pipe can be inserted at an easier location even though the pipe won't forever occupy the extra holes.
If all else fails, cut the pipe into sections small enough to be inserted and use couplers to re-join the pieces.