Magtube Spring, very confusing situation.

[easy]

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I think you're right insofar as you can remove the follower and the gun will still feed shells (without a follower the spring will enter the receiver, but I can still imagine a CFO busting balls over that).

Not if you insert a shell into the mag tube first, then install the spring and cap on the end.

Would this qualify as permanent do you think?

Yes. Fill the extra space at the end of the extension so to limit it to no more than 5 round the size the gun was designed for.