Things didn't go as planned.
Florescent light protector tubes have an unfortunate tendency to curve inwards when cut. I discovered this only after I had already cut a slot halfway down one side of the tube. The original design had only one support ring in the middle, which was made from part of the same 10-round vortex mag that provided the feed lips and cap, like so:
Making these support rings, and ensuring that stray epoxy didn't impede the motion of the follower, turned out to be a very tricky business. The guide tabs had to be ground to make them narrower. Cutting under the green plastic pieces was a little awkward, the space under each one had to be cleaned out with a file, and during this process the green plastic often broke away from the epoxy. Superglue was used to fix it each time.
Overall, this was a laborious, finicky, difficult build, and as such this design is not suitable for mass production.
Oh, and there is one more problem: the support rings don't actually help! They hold the tube in place enough to allow the follower to slide fairly smoothly (with some, but not unreasonably much, friction - note that the follower does not slide when the tube is at an angle, as it is in the first picture) - but vortex disks have grippy foam edges, and as such do not slide through the tube without the application of considerable force!
|This tube has curves in all the wrong places.|
Finding another type of tube of the right size which is not heavy would be difficult - FL tubes are pretty much perfect for this application except for their tendency to curve inwards. Attempting to fabricate a structure from scratch would also be difficult - let's call that idea a last resort. I could use an intact FL protector tube and trim the guide tabs from the follower, and use magnets in the follower and the bottom of the magazine to suspend the follower at the right height. This would result in an dramatic increase in feeding force as the follower approaches the point where it stops - and excessive feeding force may result in a jam. Cutting lots of slots in the FL protector tube, such that no unbroken section of tube spans a large part of the tube's circumference, could reduce the tube's inwards curvature, but might make it fragile - this is at least worth a try. Finally, I could coat the FL protector tube in something which becomes rigid when it sets. A fiberglass repair kit might work well here (so long as the magazine can be made without having to grind fiberglass - fiberglass dust is nasty stuff). This seems to be the most promising idea, and this is what I intend to try next.