brass is a mixture of copper and tin, tin makes it harder copper makes it softer, the exact mixture, combined with chamber size, and how much pressure you load them with is the key to brass life, if not given room to stretch in the chamber, it won't until your pressures exceed what the brass itself can withstand, this is why custom chambers will exceed factory ones in MV but not show signs of higher pressure. your chambers actual size compared to your die setting has more to do with pressure than anything when your looking at case life. seating depth is also a major culprit, you can be shooting under max velocity loads but still over pressure due to pressure spikes from seating too long.
RR