Christopher, 300 rounds or so then a good cleaning will tell the tale of overall reliability. If you experience issues after this, then it may not be a break in issue with the pistol. Chambers are now more fully supported than past models.

it should not require a break in but may need a good cleaning to remove any factory applied preservatives. it has a fully supported chamber

Christopher, 3-P320's I own shot perfect right out of the box. Cleaned barrel, lubed slide rail shot thousands of rounds no issue. Do not know about more support for casings. My rounds all chamber up to the small boat tail at base of casing. Couldn't really be more coverage.