Originally Posted by Virginian
It is probably functioning exactly as designed. When the hammer drops, the action is unlocked. Otherwise, what do you think unlocks the action so you can chamber another round? During recoil the inertia of the forend, bolt, etc., keeps the breech closed, but as the gun stops the action opens. My Wingmaster does the same think with any load if I don't hold on to the fore end. Winchester claims their Speed Pump is faster because it does it, too.
The 1200,1300 and Speed Pump Winchesters do not have an inertia slide lock like the older model 12 and current 870 Remingtons and 500 Mossbergs. The inertia lock was designed to keep the action shut in the event of a hangfire. By the early 1960's hangfires had become virtually unheard of, so the Winchester 1200 pump was deemed not to need an inertia action lock. All the Winchester 1200, 1300 and Speed-Feed series slide actions will open and self eject when fired without holding the forend.
Regardless of make, you can test your slide action shotgun for the presence of an inertia action lock by holding the forend firmly back while pulling the trigger on the UNLOADED shotgun. If the slide stays locked, your gun has an inertia action lock.
The inertia slide lock is only important if you have a "quick" hangfire measured in miliseconds. Winchester engineers determined that by the 1964 introduction of the model 1200, such shotshell ignition problems were virtually unheard of.
In ordinary use, the recoil of a fired round causes the forend of any slide action to move foward and unlock, the shooter will never notice the function of the inertia slide lock in those shotguns that have them.