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One technique of joining plastic across cracks is to have a number of tiny threads of steel wire cable, like 5mm long. Lay them across the gap, and touch with a hot soldering iron till they "sink...

On the subject of magazine spring sourcing:

https://m.aliexpress.com/item/4001079872684.html

204758

Easier still, jump-start with some 1/2" schedule-80 seamless steel pipe and create an ugly smoothbore .58" musket.

When John Browning sat down to design a gun, does ANYONE here believe he would waste his time on designing complete sets of unique springs?

No, there were and are engineering catalogues of...