Have you given the gun a cleaning? You can bend an aluminum cleaning rod into 90 deg and use a 38/357 wire brush to scrub the chamber. Take a tooth brush with Hoppe's and scrub the bolt face around the extractor claw. You can look at the rims on the cases and see if there is a difference between the 2 brands. Take a picture of an empty case rim that was giving you problems. How old is the rifle? How familiar are you with the mini 14 and shooting?
Remember the battle of Athens, TN!
"Work as if you were to live 100 Years, Pray as if you were to die To-morrow." --Benjamin Franklin