I just de-dimpled four 870 Expresses a few months ago.
I tried that socket thing and never did find one that fit the mag tubes well enough to suit me. A scavenger hunt in an old machine shop produced about 6" of 1" bar stock which had been turned down to 0.872" on one end. The smaller diameter will work for driving down the tube to knock the detents down. I then shimmed the bar for a tight fit in the mag tube, drove it in again and started to work hammering out the detent opposite the shim. I use a flat punch a little bigger than the detent itself. Don't get in a hurry. You'll see the dent raise as you work.
Once the dent appears to be fully raised, reverse the shim and raise the other detent in the same manner. When both are finished, feel around inside the tube... if you can feel the slightest bump in there, it is enough to inhibit free passage of the follower. Here's where the sanding drum comes in. Cork the mag tube with a paper towel, etc. so your grindings remain in the end where the detents were. Sand until you can feel absolutely nothing where the detents were. Poke the paper towel out from the receiver end to get rid if the grindings, apply a little cold blue and scrub the tube again. Finish with a coat of oil and you're done, except field testing.
I'm going to turn the bigger end of my home made 'dent raising tool' to an exact fit for 870 mag tubes, less 0.005", to get around using the shims. I worry a little about getting the tube out-of-round and a closer fit for the mandrel should all but eliminate that possibility.
Another good thread on de-dimpling
here.
