on a 15-22 .625 is the norm. ar-15s have less length around .5 usually.. i put .625 on them and recrown the barrel this needs to be done because of the factory recessed crown is allmost the same dia. as the .5 threads and that leaves a thin lip that can be very easily damaged... make sure that he indicates the bore of the rifle in not the o.d.
if I can help in any other way let me know
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07/SOT2
NRA Instructor
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