Have you tried pushing the bottom of the mag towards the muzzle and cycling the action? Your feed ramps on your barrel extension are what my money is on right now. Try pushing and holding the bottom of the mag forward (towards muzzle) and cycle the bolt, releasing the charging handle at the extreme rear of it's travel. If it loads rounds correctly in that manner, see if it will cycle correctly being fired in that manner. There is a possibility of your mag well angle being incorrect, but it's so slight that it's almost impossible.