Shotgun choke

Question
Is a shotgun (in this case, 20 gauge, manufacturer most likely Mossberg) EVER SOLD with the slug choke in place?  If not, is it legal for a firearms dealer to CHANGE THE CHOKE in the store if purchaser requests it?

Answer
I'll try to answer your question.  Forgive the lack of salutation, but you did not provide a name.

First, there is no such thing as a "slug choke".  

The most frequently encountered shotgun chokes are either 'improved cylinder', 'modified', or 'full' chokes.  Many shotguns have variable chokes, but generally a choke tube is installed inside the barrel of the gun at the muzzle end.  The purpose of the choke is to provide a constriction at the end of the barrel that will compress the shot as it exits the muzzle to help keep the shot pattern tighter.  The three different chokes above each have a different diameter, with the 'full' being the smallest and the 'improved cylinder' being the largest.

It is possible to shoot a slug through a choked barrel, but deformation of the slug occurs.  The tighter the choke, the greater the deformation.

There are rifled barrels available for shotgun which allow a shotgun to shoot slugs more accurately making it possible to hunt larger game with a shotgun.  Shotgun barrels are usually easy to interchange allowing a person to purchase a rifled slug barrel and change it out when they wish to shoot a quantity of slugs vs. the shot through a smooth bore.  BATFE has rules that this is perfectly legal to do.

As far as I am aware, Mossberg does not provide a version of their 20 gauge pump with a rifled barrel.  That does not prevent you from purchasing one and installing it yourself or having a gunsmith or other qualified party install it.  Whether the dealer would exchange barrels is a business decision that they would make, but there is no legal restriction.

I hope this helps you out...

Doug Little