A long time Automag fan but played mostly pump.

I bought a real old field Automag and tried to reserect it a few years ago but to no avail.
What I recall is that it leaked air down the barrel.

With a limited budget I figure a old automag is far better than No Automag!

The questions are.

1) Why is air leaking down the barrel?
2) How do I fix it?
3) What to do next?

Thank you for any help you can provide!