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Anyone make an epoxy or similar product that has a low melting point? That would make it possible to melt out of the mag later.
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You could also consider a product called CerroSafe. It is a low melting point metal alloy. You can pour it into the magazine and fill the space. It will harden into a metal plug/block. You can melt it and pour it out for maintenance of the magazine. I think the issue is going to be cost, if you have as many magazines as I do.
Epoxies lose their adhesive strength and begin to decompose at about 350 degrees (F) and above. A true epoxy does not "melt". Epoxy is a (two part) curing resin system, more of a thermoset rather than a thermoplastic. There are plastics that do melt but epoxy isn't one of them.
What you are referring to is called a thermoplastic - a material that melts with elevated temperature, resolidifies as it cools, can be remelted,... PVC, styrene, .... there are so many.
Those are the materials. Then there is process (how to apply the material).
Perhaps one way to go is purchasing and installing a commercially made magazine block, then melting it in place to make it permanent.
None of the above are likely to work if you have a plastic follower and/or base pad, as those items will get whacked by the heat. Luckily, metal base pads and followers are available for AR's.
Alternative method - drill small hole near base of magazine body; insert metal tube of sufficient height to limit capacity to ten rounds; match drill hole in tube; rivet tube into magazine. Install base pad.
There are so many ways to do this it boggles the mind. When magazines cost only $20, the problem is doing it cost-effectively. Also, with tens or hundreds of magazines to do, the individual is faced with the issue of total labor hours required to do the modifications.
The problem is the need to do it, at all.