[naminator]

Ok:

1.) I cant comment on the mag problem. No idea why they would backout like that.

2.) What do you mean the accuracy is lacking? Arr you get 20 feet out of the gun? Or are you getting 100 feet. It is a single shot spring shotgun, however it is one of the more usable shotgun platforms. We need to know what you are trying to get out of it. About the grease, have found excess grease?

3.) google g&p m870 diagram. 4 or 5 diagrams popped up immediately.

4.) The m870 already shoots a bit hot. You might want to think about increasing the weight of your bb first. Sometimes it is simply a matter of finding that sweet spot.