AFAIK they never made a bolt action shotgun
No, the Mossberg 395 is a bolt action shotgun, NOT a semi-Automatic shotgun. It is a fine bolt action though.
A bolt action shotgun in 12 gauge.
I'd stay with standard velocity ammo which it was designed for.
No. If it does not have 5.56 on the barrel then use .223 ammo.
The Mossberg model 185D bolt action shotgun was made from 1947-1959.
Some bolt action shotguns had an adjustable choke, and it is generally NOT recommended that slugs be fired through those. If there is no adjustable choke at the muzzle, your shotgun should be fine. Stay with rifled slugs.
Mossberg, it is their M#85 ( I'm assuming that this is a bolt action shotgun).
Model 58 is an automatic not a bolt action
Since I don't know what brand of shotgun, the generic answer to finding parts for many older firearms is Numrich http://www.e-gunparts.com/
The value of a 410 Mossberg Model 183T bolt action shotgun is between $150 and $300. The overall condition of the shotgun plays a large part in its value.
Models 583-13 through 583.22 are under a safety recall- unsafe to shoot. Sears will give you a $50 gift card for sending them the bolt of the shotgun.