Standard factory choke on a smooth bore "slug" barrel is none. Cylinder bore.
Standard advice (for generations) is that slugs work best from a cylinder bore. Now, we are talking the traditional Foster type "rifled: slugs and smooth bores here, not more modern designs or rifled barrels.
True that slugs are NOT spun by their "rifling" and stay on target because they are point heavy, like a dart.
The "rifling" on the slug is not for spin, it is there to allow the slug to compress and pass through ANY choke without serious deformation or damage to the gun. SO, you can shoot slugs through any choke, but you usually get the best results (accuracy) from the most open bore. Usually.
Considering smooth bore and only a bead for a sight, "accuracy" from a regular shotgun is not impressive, but minute of deer at 40-60yards is something most will do.
Rifled bores, with rifle style sights give the dedicated slug gun advantages, which translates to more easily usable range, generally.