You're still only getting one shot per trigger pull, so it is legal. It's just not found on many modern shotguns.
As for useful, it is about as useful as hip firing at best, and less so more than likely since you are hip-firing rapidly.
slam fire is when on a pump shotgun, you keep holding the trigger down while working the action- on many older shotguns like model 12's and m37's the gun will fire as soon as the action closes, which allows you to fire as fast as you can slide the slide back and forth. It's a good way to have fun shooting off a magazine of rounds as fast as possable, and not of much other use.
years ago I had a Ithaca M37 police. Parkerized with extended magazine and it did not have the trigger disconnect. Several times at the range I played with the "SlamFire" feature but was never able to get through a complete magazine without a jam. I ended up swapping out that shotgun for a 870.
the M37 does have a disconnector. If it did not, the hammer would follow up behind the bolt and not fire the ammo. The hammer MUST be held back till the bolt is closed. The moment the bolt closes it trips a sear and releases the hammer. If you hold the trigger back and close the action very slowly, you can hear it click just as it releases the hammer.
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