If the factory said the brass in the case was off spec., and a gunsmith checked the rifle (Go / No Go Gauge?), then you should be all right. I have always found it is normal for a rifle to chamber a fired casing that was fired in that rifle. A blown primer is usually a sign of high pressure, but can happen on a case that is to short, and is driven into the chamber by the firing pin before firing, allowing the primer to back out. That seems unlikely on a case with a belt however. Possibly a slightly long or damaged firing pin? Are the primer strikes in fired cases deeper than usual, or have a sharp edge to them? I would take a fired case, and compare it to an unfired factory round, and see if there is a noticeable difference. There will be some, as a fired case shapes itself to the chamber, but it should not be excessive. If in doubt, make sure the gunsmith did use the proper chamber gauges. In situations where the case was too short (shoulder), I have seen cases split in half, but not blow their primer, though I am sure it is possible.
Larry