Lengthening the forcing cone has nothing to do with the guns chambering. The forcing cone is an abrupt level of decreased diameter at the front of the barrel that forces the shot/cup down the barrel. When the forcing cone is lengthened the shot is no longer forced at such a sudden pace. The process is more gradual. It is also very important that the forcing cone, after being lengthened, is polished to ensure shot pattern density. If you are looking to make your gun shoot 3 1/2" shells, you need to be looking for chambering services. This can usually be done by a custom gunsmith, given the gun make and model.