You should be able to slide the BCG into the upper with light fingertip pressure - there should be no increase in resistance as the key encounters the gas tube. You're likely to have FTFs since the bolt probably won't close, but there shouldn't be a KB. Either the key or tube is out of spec or there's some crud interfering. You can clean the outside of the tube and the inside of key with a cotton swab wet with CLP or your solvent of choice. If you still have problems, measure the diameter of the mushroom on the end of the gas tube; it should be 0.186" IIRC, the same as the full-diameter portion of the tube. If the tube is the right diameter, the problem is the key. If yoou don't want to return the offending part, you can reduce the OD of the mushroom with emery paper (work carefully - you want it to be round, not elliptical) or open the ID of the key with a drill bit, but the vendor certainly won't repair/replace/refund if you've attempted to modify it yourself..