If you consider the various functions of the firing sequence of a typical pump action shotgun you'll understand what is happening:
- Fire
- Set disconnector, unlock action
- Retard slide
- Extract
- Eject
- Cock hammer
- Release round from magazine
- Advance slide, lift/position round
- Chamber round
- Lock action, release disconnector (activates the trigger)
- Drop hammer at full trigger pull
- Repeat the cycle.
What happens when you short stroke the action (it doesn't take much) is you by-pass some, or all, of the functions between unlocking the action and re-locking the action. With the action locked, the only way to release it is with the action unlock button or drop the hammer; but, since the hammer is already down (short stroking didn't fully cock it) that option isn't available.
As
the rifler mentioned, how you grip the fore-end is a significant factor in the gun's operation. A improper grip can contribute to cycling problems, like you've experienced, and increase the felt recoil (kick) in your shoulder and cheek. At times like this, it's easy to say, just follow the recommendations in the owner's manual. With the M-500 the manual recommends:
With the firearm pointed in a safe direction and held in the proper firing position, move the safety button fully forward to its “OFF” (FIRE) position. Pulling the trigger through its full travel to the rear will cause the firearm to fire. Ensure muzzle is pointed in a safe direction before placing finger on trigger. They cover the safety aspects of firing; but, they don't give you a clue about what is the
proper firing position. The Remington 870 manual is worse, it doesn't even mention a
proper firing position.
You mentioned the problem occurred while shooting some Skeet. Proper Skeet shooting techniques will almost guarantee that you won't short stroke your pump gun. In Skeet (and the other clay sports) an important aspect of breaking targets is a smooth follow-thought. After pulling the trigger keep your head on the stock and keep a smooth swing, seeing your lead, until you see the target break. Some better shooters owe their success to an exaggerated follow-thought. The pump shooter can follow-thought on single targets as well as any other shooter. Many pumpers don't retard the action until they've dismounted the gun and have their hand over the port to catch the ejected hull.
With doubles, newer shooters often lose their concentration and immediately think about the second target as they are pulling the trigger on the first one. A poor, or nonexistent, follow-thought has caused the loss of many first targets in doubles. It's even worse for pump shooters. They are often trying to cycle the action when they should be smoothly following-thought on their first target. What happens is they not only frequently miss their first target, they jamb the action in the process. Follow-thought on that first target! After you see it break, then reverse your gun and cycle the action. Until then, keep a firm grip on the fore-end. With enough practice it will become second nature. My Skeet mentor, an aficionado in doubles, has a saying: You have all the time in the world, but don't take it.
Some shooters advocate maintaining a slight rearward pressure on the fore-end when shooting a pump gun. They argue this will facilitate a faster reloading time. It may work for some shooters and not for other; but, in either case, simple physics says it will contribute to the gun's kick. In Skeet shooting, don't think about a fast reloading cycle until after you've followed-through on your target. I recommend maintaining a firm grip on the fore-end with a slight forward pressure. This will reduce kick (a fraction) and keep the action closed while following-through. YMMV
