Cans-R-King

I inherited an older H&K model SBE with a feeding problem. The gun extracts and ejects the fired shell fine. The next shell is released from the magazine, but sometimes jams when the bolt tries to push it into the chamber. The front of the shell appears to be catching on a slot in the barrel extension and the bolt partially crushes the shell so that it will not chamber fully. This happened with different ammo types, both 2 and 3/4" and 3". The gun is very clean and well oiled. I have polished the magazine tube and chamber and have cleaned the recoil spring tube. Shells slide freely into the chamber and there are no obvious burrs on the edges to the slots in the barrel extension. I have replaced the magazine spring and follower and the carrier spring. The situation improved when I replaced the recoil spring with a Wolff extra power spring, but the problem has not been eliminated. Seems like the shell is not in line with chamber when the bolt tries to push the shell forward. Any suggestions would be appreciated. I am a new member so I apologize if the problem has been addressed previously.