But twenty-four inch barrels alone on a pump action or self-loading shotgun won't alone, as you know, since 1988 keep it out of s5. The notable gun that fell foul of this was the Mossberg 500 guns that were fitted with bullpup conversion stock. This made a gun that was UK s2 legal before 1988 then current law was merely that any smoothbore gun with a barrel over 24" was a s2 shotgun regardless of if side by side, self-loading or pump action or indeed magazine capacity or if the magazine was detachable. In 1988 an overall length was introduced of 39" on any self-loading gun or pump action gun (even with a 24" barrel) that regardless of magazine capacity placed it under s5.