I can see why a 3.5" chambered barrel would be ill advised being put on a 3" receiver but there are many shotguns out there actually designed to accept 2 3/4- 3.5. If your receiver is set up for 3.5, I just can't wrap my head around why they so strongly state NOT to use a 3" barrel on it. Maybe Mercy will chime in here with some techie jargan that will most assuredly give me a headache