I took mine out last week. A decade+ of clays, duck, goose, dove, quail and upland...spring was rusted (its been swimming a few times). The "mag tube cap" (using Beretta parts names) is screwed in. The "mag tube cap spring" secures the "mag tube cap" in place preventing it from un-threading. Take the gun apart, then use a punch to push the "mag tube cap spring" in and use a screw driver at the in of the "mag tube cap", it is cut at the end for a screwdriver. Once you get a turn start the spring will no longer need to be depressed. The "mag tube cap spring" will look like a pin coming out of the mag tube. The The mag spring is under pressure - but not too bad. I have a plastic orange plug. I leave mine in. But that is how you do it.
Some say you can toss the "mag tube cap spring", but I worry that the "mag tube cap" will un-thread when removing the "fore end cap".
I ordered parts from Coles Gunsmithing (colegun.com). They took care of me.
Tim