Inertia lock test.
The 1200,1300 and Speed Pump Winchesters do not have an inertia action 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 pulling the forend firmly back while pulling the trigger on the UNLOADED shotgun. If the slide stays locked, your gun has an inertia action lock.