A lot of the accuracy potential will depend on the consistency of the alignment between the breech and the insert.
It also sounds like the inserts may be intended for slug guns where real sights are frequently found. If so, then the opportunity for inexpensive and comfortable practice is obvious.
Now, having speculated a bit, a question arises -- could one wingshoot with a smoothbore slug gun to good effect? If so, would that add to the flexibility of the 1-gun survival tool? The idea being that the 22 LR for small game, quiet engagements, the slugs for larger game out to 100-150 yards, and birdshot for flying or fast moving game.