Ah, just a tip. Even though you said you had removed the mag tube once before ..........
Often, the ferule that retains the mag tube has burrs where the pin goes thru. The burrs will often score the mag tube. Remove very carefully and before you re-assemble, inspect and polish the inside of the ferule. DO NOT twist the mag tube back and fourth to remove
it. You'll get some really nasty score marks.
Coffinmaker