First of all recognize that primer flattening might or might not be a sign of overpressure. There are something like 25 different ways to determine if you are overpressure. Looking at the condition of the primers is just one.
My own personal experience is that I have seen flat primers with factory loads. This can be the result of at least 2 things. The first is factory tends to be loaded rather warm, so it is close to max pressure. But with their larger blended lots of powder it is safer for them to do so than you. The second and I feel this is more significant is that factory rounds tend to have the shoulder set back pretty far. This is so they will chamber in basically every chamber out there. The more headspace you have the more the primer backs out before the head of the case slams into the bolt face. This will make the corners of the primer flat even with moderate pressure.
My suggestion would be for you to learn the other 24 ways (signs of overpressure
) and rely on a combination to determine if you are approaching maximum pressure.
Brass flow into the ejector opening and hard to lift bolt handles are more positive signs of overpressure than primers looking flattened.