Having spent the afternoon tinkering in my basement shop, one of my chores was to un-compress my low-cap springs. This gives them better feeding and no loose BB's left in them.
Anyways, the internals are amazingly simple, abeit, the odd shaped spring (a rectangular spring...). It would be interesting and fun to make a high lowcap. I figure if someone has a proper router bit, they could make spring fed box mags that can hold 500+ rounds without too much difficulty. PG has Redwood plastics and getting some teflon-mix plastic is cheap and easy.
Simply router out 2 half circles, use a long round spring, and double, triple, or quad stack. Maybe there are jamming issues if they are stacked anything higher then double alternating... Still, to a triple alternatiing, a sprial effect, and there will be no jamming.
Finally, imagine a box mag that does not require manual or elecrical winding, would that be nice not to have the electric whirring? Just feed some a standard loader and you are good to go.
-Doug
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Jeez, you are right... that would be an annoying amout of loading...