They’d have to be pretty old to be shrink to fit. Those aren’t made anymore. I’ve got a few of them from the early 90s.
Yeah, I think you will have a had time using a pocket holster in Levies 501s. Shrink to fit was those old style jeans that were so dark blue they were black...and the material was really stiff and coarse like a canvas tarp. After the first wash, the material would shrink up, turn dark royal blue and become soft. You had to buy them a size larger than you wear.