My grandfather bought a Savage 99 the first year it came out in .243, even found the original sales slip and tags for it in his desk at my grandmother's house. Sometime in the 1950's, he used it to kill deer until he died in 2005, around 50 years of use. That is an heirloom, a cherished piece, even if I don't shoot it anymore. I shoot his High Standard a lot, something he shot, so heirloom. The old Winchester pump 22 that was the "pig gun" for killing hogs for butcher is my father's that I take care of, fourth generation with me. is definitely an heirloom. An 1897 Winchester is an heirloom from my other side. along with a "patina" classic Glenfield bolt action.
I inherited a Winchester pump from my grandfather, one of the cheap junkier ones, that he almost never used. He bought a Winchester bolt 22 he never got around to putting optics on, never was used. Such guns aren't heirlooms, in my opinion, because even if there was ownership there was no usage and no real history. So its easy to understand OP's and other's points on the issue, even within a collection you inherit you may see some as family keepsakes, others were just guns that happened to be in the safe.