Remove trigger group from the 870/1100.
place strainer in sink.
boil kettle of water.
place trigger group in strainer.
wet trigger group with warm or hot tap water.
scrub trigger group with old tooth brush.
add favorite dish soap to tooth brush and scrub again.
when kettle has boiled pour over trigger group.
remove hot trigger group from strainer and place on a towel to dry or in the dish rack at edge of sink.
the trigger group will air dry in short order.
lubricate trigger group with G-96 or other choices.
re-install trigger group and you are good to go.
Tight Groups,
Rob