As for building one's own 870, done it,and I'm tool challenged.
The last one started life as a tower weapon for the Md Pen.When they closed the Pen(First used as a prison in the Administration of T Jefferson!), a gun shop I use recieved many very used 879s. Some were stripped for parts,and the receivers refurbished,converted to 3 inch and so on. I bought one for nostalgic reasons(retired from Md DOC) and built a not pretty but excellent bird gun. I had a 21 inch tubed bbl left over from another 870 that had an extended forcing cone.The matte finish on that vent rib bbl matched up OK with the Parkerized receiver.
The shop threw in one of the most ugly, nasty,beat up misused pieces of walnut ever to be seen. After a little work and some flat black spray paint, it didn't look so bad. Some judicious work with aluminum foil gave me stock measurements with a little cast off and less drop than standard.
Forearm was the original corncob type from my first 870,bought by Pop around 56. A trigger group with an acceptable slap trigger was $35. The mag spring from that 870 was also used, it's only 40 some years old and works fine(G). I did get a steel follower, don't trust the plastic and newer stuff from DOC experience.
The short bbl moved balance a little far back so I hogged out about 3 oz of wood from that ugly stock under the pad where it wouldn't show. This balances between the hands nicely and seem to swing like a lighter weapon. Weight is on the light side, maybeso 6lbs,10 oz.
Oh,yes, it seemed a little slow after hogging out that wood,so I took the mag cap and belt sanded it down from a domed shape to flat and installed a swivel stud.This took another oz off the front and sweetened the swing. While it's not the prettiest shotgun on the clays range, I do OK with it.
And, while this was a bird gun rather than a defense piece, the process is the same. For a HD 870, I'd farm out mounting a peep and something better than a alap trigger, but the rest would pretty much work at the old kitchen table.
And while I like rust resistant stuff myself, there's a fair to middlin number of weapons here at Casa McC that were made in the 40s and 50s, been used close to the Chesapeake Bay and the Atlantic frequently and are rust free.
Hope this answers all questions, if not sing out.The only dumb questions are those that don't get asked....