I have had a 303, 390, and a 391 over the years. They were all great guns. I'm not sure that I ever heard of a break in period for the guns, or what to do, but I can give you a little advice. Mine were great guns, and they would cycle any kind of ammunition I loaded into them IF I kept them clean and well oiled. Being gas operated means that when the gas port into the loading/ejection mechanism gets some carbon build-up in it it will become less effective. I always cleaned the guns after 300 or so rounds because around 400 they would stop cycling light loads. Just something that I though I might pass on. Enjoy! They are great guns!