WHAT DOES BERETTA ARX100 FOR SALE MEAN?

To break in The ten round magazines, I only loaded five rounds at a time. I shot 20 rounds before my first jam. Some rounds later on every single shot was jamming. The expended casing failed to eject plus the slide was half open up with a half seated clean spherical. By pulling the slide again the round would eject and I could fire again. I tried l