Yes even the hard cast slugs, flare out just enough, when you fire the slug. Actually this happens in the shell in the chamber. I hope you are using hard cast slugs, because the soft slugs deform enough to cause high chamber pressures, and can be very inaccuate.
Shooting a slug into a large box of clothing, packed very tight, can help you recover the slug in very good condition. If the skirt is pushed out and forward .... The slug is to soft. The skirt should look the same as before you loaded it. But a dial calipers with show, that it did push out just enough, to grab the rifling in your barrel. The wad should show that it did grab rifling as well.
If you do fill a box full of clothes .... You may not want to use your wifes clothes. She may not appreciate having slug holes in the @ss of her blue jeans

Although, jean material, will stop a slug better than her panties will
