M2 and M3 had their large 12-inch guns removed in the late 1920s to conform with the Washington Disarmament Treaty, which stated that no submarine should have larger than 8-inch calibre guns. M2 was refitted with a seaplane hangar forward of the conning tower and a catapult to launch a small Parnall Peto seaplane. This conversion was a success, and M2 could surface from periscope depth, open the hangar door, catapult the plane, close the door and dive again within five minutes. She was subsequently lost during exercises in the English Channel in 1932 when her hangar door was left open.
In 1927, M3 was converted to an experimental minelayer, stowing her 100 mines on rails inside a large free-flooding casing outside the hull. The mines were laid over her stern by means of a chain-conveyor belt. M3 was finally taken out of service in April 1932, and scrapped in 1933.
Although the M Class may be considered unsuccessful because their big guns were never used for the purpose intended, they were popular with their crews and were claimed to be handy under water, quick to dive and easy to handle.
