no
No. The Model 66 does not have user-interchangeable barrels. Your existing gun could be modified by shortening the barrel, adapting for choke tubes and installing rifle sights. sales@countrygunsmith.net
You can try Hasting's. They make after market barrels.
You did not specify the make of your Model 300 (there are several), but in general, a 12 g shotgun CAN fire slugs from any standard barrel. You will get better accuracy from a slug barrel, but you could use any barrel OTHER than an "Extra Full", or a shotgun with a variable choke.
No
Yes. Slug barrels have a different "Choke" then shot barrels that make them more accurate when firing slugs, but you can fire slugs with a stock 1200 barrel.
Not anymore but I hear you can take a 1300 barrel and have gas ports added and that should work.
At this time no,there are not any available.You could have a model 1300 slug barrel redone and have the gas ports added to this barrel.Then it will work fine in your model 1400 Winchester shotgun.
Yes.
I have one that I would sell.
Depending on condition, $325-$375
yes
not a good one .