I have the P3500 Steady Grip in Mossy Oak Obsession and really like it BUT there were some issues. First, my gun has a 100% fail to load rate with 2 3/4" shells and most 3" shells. It handles 3 1/2" shells fine after one modification. With the smaller shells, it double feeds out of the magazine tube, they jam in one after the other and then it can't lift a shell to the chamber. It did it once or twice with 3 1/2" until I changed the magazine spring. I ordered a spring for the M3500 which was longer but had a smaller wire diameter. Then I cut the new spring to the length of the original spring and installed. Since that time I have not had a single issue loading 3 1/2" shells. The smaller diameter wire creates less force and is working well.
Secondly, even if I only put one 2 3/4" shell in the magazine and cycle the gun. The lift brings the small shell up at an angle that will not allow the bolt to push it into the chamber. This is 95% of the time. This also happens with some 3" shells. It will lift 3 1/2" shells perfectly and loads them well. I consider my gun to be a 3 1/2" gun only.
This is also a big gun. The receiver is 2-3 inches longer than my Remington 870 Super Mag. The gun feels long and big, even with the 24" barrel. Having said all this I really like the gun. The action is smooth and solid. The POA/POI is spot on. I did change the bead from the OEM bead to a TruGlo Long Bead only because I like the smaller dot. I also installed a MCarbo sear spring which works really well.
Maybe the issues I had where just with this one gun. I haven't read about anyone else seeing these problems. Hope this helps, I saw your post and thought I'd share what I found. Good luck.