The easy answer is when the results downrange degrade for no known reason. I have had a personal AR that went from "typical" groups to a catastrophic "buh-bye" in terms of accuracy during one session of around 200 rounds. In fact, 1 30 rd mag started working as advertised, then after it was expended, the target at 50 yards had banana shaped holes all over the place. In the mil, M4/M16 rifles are considered "shot out" based on chamber erosion, which starts at the chamber. In theory, tumbling bullets should never happen in the mil if maintenance people are doing their job and gauging weapons on schedule. I have had my M4 "fail" chamber erosion checks while it was still grouping fine. The armorer/gunsmith would tell me it failed, and replace my barrel/bolt, no discussion. Which meant that I had to start at square 1 setting up my M4, zeroing, break-in, etc. I have also seen sniper rifles like a M24 with an unknown round count go in 1 day on the range from shooting MOA or better groups to printing groups that looked like they were from a Mosin with a bent barrel.