8.35-inch +3B tube, cap, and spring kit that Nordic identifies as flush on an 18.5-inch Benelli M1/M2 barrel.

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Nordic's +3B MXT kit is the published flush-length tube geometry for an 18.5-inch Benelli M1 or M2. The current kit is 8.35 inches long and includes the extension tube, cap, and full-length spring. It still cannot mount to an M2 without NUT-BN-12-00. This guide also pairs it with BCL-12 to keep the barrel and extended tube supported as one system. Final shell count varies with shell length and the gun's original magazine configuration.
Add the Nordic Components MXT +3B 12 Gauge Extension Kit to your build and see how it enhances your platform.