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To permanently attach a flash hider to make a barrel at least 16 inches and legal as a non-NFA firearm, the flash hider must be welded using full-fusion gas or electric steel-seam welding, high-temperature (1,100 degrees Fahrenheit) silver soldering, or blind pinning (pin that doesn't go through on both sides) with the pin head welded over.
Barrel length is measured by closing the action on the unloaded weapon and inserting a dowel until it rests against the breach face. The rod is then marked at the end of the flash hider and the rod measured.
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