But I've also read that people have used piano wire and coat hangers as pins so
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Piano wire is fine, but I definitely would not use a coat hanger. Very soft steel, would easily shear and may not even damage the threads.
As a rule of thumb, I'd say anything you can easily bend back and forth to failure with your bare hands from a 2" length or effortlessly snip with a pair of $3 side cutters is not sufficiently strong.
Just remember that whatever it is, it needs to render your flash hider non-removable with common, conventional means (wrench, channel locks, vise grips, etc). If ATF were ever to test it, they will put 200 ft-lbs to it, and it has to either take it or destroy something. If they're able to break it loose, but the barrel threads are destroyed in the process, it was permanent. If they snap the muzzle device, it was permanent. If the barrel twists or spins in the receiver, it was permanent. But if it comes off with no or minimal damage to anything (i.e., you could screw it back on or easily clean up the threads with a thread chaser), then it was
NOT permanent.
Obviously having to drill it out, split the muzzle device or take an acetylene torch to it is above and beyond common means of removal with hand tools.