Yes the Winchester model 12 is a pump gun not a single shot unless something is wrong with it to make it a single shot. If that is the case I would not use it.

I don't know all that much about all the new pellet materials but I do know that you should not shoot steel out of that gun. They did not have the steel shot requirements in 1948 that we have today and as such the barrels and the fixed choke won't take the abuse that steel dishes out for very long.