Re: JC Higgins model 20 (High Standard model 200)
The Sears Model 20 was made by High Standard in several versions between about 1948 and 1963. The High Standard model 200 did not appear until after 1965 and is similar to teh model 20 nut not all the parts are the same.
THe Sears ID number 583.?? can provide some indication of age and variation. For guns made December 1956 and later, there will be an assembly date code on teh barel just forward of the receiver.
High Standard pump shotguns from the early years had "lock aligned" barrels and were not intended to be removed by anyone but the factory or competent gunsmiths with the proper tools.
I don't know what magnum shot is but if you are talking about steel shot, the answer is "do not use it".