Are all recievers standard tapped? I was looking at a weaver rail and it said it was for a mossberg...but I can't seem to find one for a remington.
No... Need to find one for your particular gun..
The standard 1 piece weaver rail for 870s is 429M. I believe only super Mag guns come directly from the factory with a drilled and tapped receiver, so if you have a 3" gun that is drilled and tapped, it was done later in life and could have used a number of different rails that "will work" but not necessarily built for an 870.
You can get a blank picatinny rail and cut to size. Then drill your holes to match the holes in your receiver. I just did it with my benelli so I could custom fit it to my gun. I'm very pleased with it. I bought it on Amazon for $9.50 shipped to my door.
Jeremiah 29:11
The 3" turkey model, within the last 3 years or so is tapped from the factory. you will need the correct base. Have not been any around.
But buddy finally found one at cabelas for his.
I put the Millet base rail on mine to give more adjustability for my red dot. Weaver has a cheaper priced one.
http://www.midwayusa.com/product/357224/millett-1-piece-picatinny-style-base-remington-870-pre-drilled-receiver-matte
http://www.midwayusa.com/product/266217/weaver-1-piece-multi-slot-weaver-style-base-remington-870-11-87-matte