So to answer the first question, "Does the ATI collapsible stock made for the Rem 7600/7400/740/760 fit the Remington 870 20ga. express?". Yes!
How does the finished install look? OK. I hope the new stock ATI is marketing will fit and finish a little nicer.
Gun before install.
The install looks fairly straight forward. Take off the old stock, install the new.
You will notice you have a stock adapter, pistol grip, stock screw, stock and stock pin. Remove the current 870 stock, and place the stock adapter in place. On my gun, it would wedge in to the back of the receiver but would not seat flush. I figured with the addition of the pistol grip then the stock screw/washer I could tighten it down and snug everything up tight. That might have worked if the screw was long enough to start. It wasn't.
I had to take a small flat piece of steel bar stock, drill a 3/8" hole in it, put the stock adapter in place and use the stock screw and washer through the bar stock to pull the stock adapter up snug. Have a rubber mallet handy to tap the stock adapter wherever it is not seating flush. By tightening the stock screw and lightly tapping the stock adapter, I got a nice tight fit. When satisfied, remove the stock screw and bar stock. Place pistol grip in place insert stock screw and washer and tighten.
Be careful/take your time! It looks like you could strip the stock screw or the threads in the receiver if you put too much torque on this screw!
Looking into the pistol grip at stock screw.
The finished install.
Fit issues, It could be a little smoother, but it is OK. At most there may be a 1/32" edge at the very top of the receiver.
Also, I have my doubts that you will be able to pull the trigger group without taking the stock off, but a little dremel tool work would likely fix this.
I hope this helps you out on your20 ga. build!!
