So a friend of mine has a WWII Winchester Model 1897, he tells me that if you hold the trigger down the gun will fire each time you cycle the pump.

Is it possible (legal?) modify a tactical 870 or 500 to do the same?

Legal. Win. M12, early Ithaca M37, Stevens, Savage pumps all had this "feature." M500's & 870's have a disconnect that prevents this today.

There is a couple of vid's on you tube showing a model 97 being slam fired (verry cool). A chinese company call Norinco makes a reproduction but can no longer import. Every once in a while one turns up on GB but the price gets bidded up fast.

Wanted to do that to an 870, but was told it is most likely a no-go in this state, bummer.............

Both my Ithaca 37's can do that. No disconnector. It's a little scary if you're not expecting it.

I have some brand new Ithacas that do that also.

I would not consider this a "slam fire" since it's a pump shot gun so I see no reason why it's illegal in any way shape or form. A true slam fire is when you drop the bolt on a rifle like an sks pull the trigger once and due to a suck FP or faulty disconnector and negative hear the rifle goes FA.

"Pump fire" is probably the more appropriate term to use. The bolt is in battery when it fires.