Nice! The Eliseo tube gun is a sweet rig. Any idea whose action is in it??
You should be able to use an accuracy international CIP length mag in it. I am 95% sure. They are not cheap, so if you want that extra 5% certainty go to Xring.com
and send Gary Eliseo an email. He is a great dude and will get back to you pretty quickly.
Enjoy your new toy