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You're refering to rimmed, and rimless cartridges. To answer your question with the example of 9mm or .45ACP, the cartridge seats fully forward on the mouth of the case (the crimped lip around the seated bullet).
The rims on both rimmed and rimless cartridges serve several shared purposes. In a rimfire gun, it has an internal cavity with priming compound to ignite the powder charge. Otherwise, they serve as a purchase point for the extractor to hold the cartridge in the breech face, and for extraction. It is also (usually but not always) the surface the ejector acts on to expel a fired case or live round from the chamber.
There is also headspace that comes into play, as mentioned. Headspace properly defined is the distance from the back of a fully seated case, to the breech face of a fully locked breech, with the locking lugs fully seated against eachother.
To put it simply, picture this: There is a tiny bit of wiggle room between barrel, cartridge and bolt. Become a ghost for a second, and reach into the loaded gun. Push the round fully forward and the breech block fully rearward. That space between (often between a half thousandth to a couple thousandths of an inch) is known as headspace. Rimmed cartridges typically seat in the chamber, or headspace, off the rim. Rimless cartridges typically headspace off the mouth (or opening) of the case. Bottle neck cartridges are usually but not always rimless, and headspace off of the shoulder.
There are also cartridges that headspace off of both areas, in both categories. A rimmed bottlekneck, such as .30-30 win, or 7.62x54r, headspace off the rim AND shoulder. .300 win mag is a type of rimless cartridge known as a belted magnum. It headspaces off the shoulder AND the belt.
For the order of when and why they both exist, we go back in time. Your cowboy guns, such as single action revolvers, and other straight wall cartridge arms are all rimmed for the most part. It isnt untill self repeating (semi and auto) firearms come about that rimless really takes off. In tube fed arms such as lever action rifles and pump shotguns, the rim serves for feeding, allowing a pair of timed interrupters called cartridge stops, to easily regulate when the rounds come out of the magazine tube.
When box mags came around, the rims got in the way, as the stacking of those rims requires a huge curve in the magazine. Those old rimmed cartridges arent entirely gone however, the Russians still use the 7.62x54r in their belt fed PK series and box fed SVD marksmans rifle. It actually holds the title for the worlds oldest smokless powder military cartridge still in service.
Delving deeper into headspace and the why behind it, and why you should care about it. . . The small amount of wiggle room is critical. It must be both
A. Big enough to close on a round when the gun has powder fowling in it.
B. Small enough that the expansion of the case under firing pressure does not batter the locking lugs of the action, or allow the solid portion of the case to move too far out of the chamber to contain breech pressure.
If headspace is too big, headspace will grow over time. This USUALLY (not always) leads to misfires, as the gap between the round and breech face increases, eventually the firing pin doesnt protrude far enough to detonate the primers. It CAN cause case head seperation and/or popped primers. What that is is when the cartridge expands so much, that in the headspace failure's case, the pressureized gas cuts the case head and the case neck free of eachother and that atomized brass case or popped primer (steel, plastic, aluminum, whatever) is sent through the action under pressure to the tune of between 20 and 65 thousand psi. Unsupported brass can liquify to an extent under that kind of pressure.
Modern firearms are designed to vent this kind of failure away from the shooter in a controlled manner, however it is not fail proof and THE REASON why safety glasses are preeched so hard. If your gun explodes, the glasses are not going to stop the bolt going through your skull. True. Those instances are verry rare. Headspace breech failures such as popped primers and blown casings are much more common and guess what? Brass and aluminum arent magnettic. So the doc whose job it is to fix your eyes has to dig for it if you weren't wearing eye pro. Have fun with that.
There are signs to look for in your fired cases. Periodically compare your casings to those of live rounds. Do they exhibbit bulges at the base of the neck near the case head? Distorted shoulders? Stretch bands at any point along the neck? Backed out primers or firing pin indents that are inflating back into the pin bore? The most common sign for .30-30s are backed out primers, due to the lower pressure nature it doesn't typically display stretch marks.
There you have it. More than any of you ever wanted to know about the breech/cartridge relationship of a firearm.
TLDR, know your guns, periodically inspect your fired casings for abnormalities, do your research for abnormalities and wear your #$%&ing eyepro.
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