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Use heavy-duty blocks to securely clamp AR-15/M16 receiver halves in a vise without damaging them when applying pressure or torquing the barrel nut. This ensures safety during cleaning, assembly, and repair. The Rynite block and insert are resistant to high pressure solvents and provide support inside and outside the upper receiver, making it an easy one-man job to rebarrel the AR-15/M16. You can also use the blocks to hold securely a mil-spec carry handle receiver or mil-spec flattop receiver in a large bench vise for easy barrel removal/installation. Using the blocks minimizes the possibility of damage to the barrel detent pin and receiver detent notch and doesn't mar the finish. However, do not use on billet uppers or left-hand uppers as this can result in damage to the upper.