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Brayden, this is the oddball Threaded 13.5 x 1MM Left Hand. You would need a 13.5 x 1MM to 1/2x28 to use popular attachments.

James G. Is 100% correct. One note however. If you purchase the SIG SRD9 suppressor, it comes with BOTH pistons for use with the 1/2x28 and the Metric 13.5 x 1MM LH. So you're set either way, for NO additional money.

1/2x28