Why?

Short shotguns don't enhance spread hardly at all. The advantage is in its concealability. You wouldn't have the accuracy when using slugs (human error and shorter sight radius). Blunderbusses had the flared barrel to facilitate loading under pressure not for increasing spread.

Assuming no choke, spread would be at the same rate. If the spread is normally 4" at a foot from the muzzle, it would still be 4" at a foot from the muzzle (MAYBE!).