[Unclenick]

I suppose you can plunk-test any cartridge, but there are a few exceptions. The idea is that if a round fits entirely into a chamber, it should drop in freely and make a plunk sound as the brass hits the headspacing surface in the chamber, thus proving it fits appropriately. But all it proves is that the cartridge enters the chamber without interference and is not too narrow for the headspacing scheme to stop it from falling into the bore. A moment's thought will reveal several cartridges will plunk in another cartridge's chamber. While it is assumed you are not making that gross error, it nonetheless proves that a cartridge could be a bad fit, function-wise, and still make a plunk sound.

For example, I can get a good plunk out of a 45 Auto round whose case is too short to headspace properly on the case mouth. This happens if you reload a case enough times. Unlike high-power rifle cartridges, the 45 Auto case shortens with each load cycle until the only thing stopping it from going too far forward into the chamber is the extractor hook. Lots of 1911s are loose enough to be headspacing on their extractor hooks even with new brass, so the fact a round will plunk into one of these chambers does not guarantee the best extractor life, and, with lead bullets, which are not hard enough to straighten out well as they start into the chamber throat, they tend to have poor accuracy and cause a lot of leading of they are headspace on the case mouth.

The best fix for a loose fit in the 45 Auto is to stop headspacing on the case mouth and instead seat lead bullets out to headspace on the bullet's contact with the throat leade. You seat the bullet out enough so that when a bullet is touching the throat, the rim of the case is within the extractor space. This can cut group sizes by 40%, IME, and greatly reduces leading because the bullets are no longer scraping themselves against the edge of the headspace shoulder in the chamber. But, with lead being a soft metal, when these rounds are dropped into the case, you don't get a plunk sound. It's more a muffled small thud.

Other instances of bullets seated to touch a throat, even with rifles, will also not make a very satisfactory plunk sound. So, there are some instances where it isn't a great test. You just have to be aware of your load and what to expect from it when testing chamber fit.