There's "zero jam", and then there's "zero jam".
Yes...ANYTHING you can use WILL eventually fail under use at some point...provided you operate it long enough, that is.
The practical question, therefore, is under what conditions do any failures occur and what frequency.
Not all failures necessarily amount to a failure of the weapon itself. A semiautomatic weapon is designed to operate within a specific range of pressures. If you use ammunition which falls outside that range, then expect problems. But those kinds of failures are NOT attributable to the weapon itself having "failed".
Here's a clear example: The AMT Automag II was designed in the 1980s to function around the chamber pressure characteristics inherent with the Winchester .22 WMR ammunition. This was recognized IN WRITING in the owners manual, in fact. The fact that it MIGHT seem to operate fine if you used, say, CCI .22 WMR would be happy circumstance. However, if you insisted on using ammunition the gun wasn't designed for, then stand by for problems...and blame yourself, not the gun.
There are many things which can affect how a semiautomatic functions, when it comes to ammunition. Bullet design (shape), bullet mass, design velocities, type of powder used, cartridge casing, etc.
Which is why it's my practice, and my advice to others, to shoot a wide variety of ammunition through any new pistol I buy. The purpose is to see if there is anything which outright sucks in the pistol in terms of function and accuracy. From there, I'll choose something (or a few somethings) which my shiny new toy seems to "like", and stick with that for several hundred rounds of happy shooting at the range (because shooting is fun).
The intent here is to find out if there's anything my shiny new toy doesn't like...at which point I will simply quit trying to feed it to my pistol. It's insane, and aggravating, to try forcing something through your gun that your gun doesn't like. SO DON'T DO IT.
From there, I shoot a bunch of ammo my new toy appears to like. No set amount, just "a bunch". Because shooting is fun.
If you don't have problems, then you can say something on the order of "my gun shoots reliably with (fill in the blank) ammunition". From this you can make an educated determination as to its functional reliability.
PERSONAL ISSUES WITH RELIABILITY:
By this I really, truly mean PERSONAL. Things which are YOUR fault, not the gun. I covered one above: trying to shoot ammunition your gun has TOLD you it doesn't like is one of them. That's a problem with YOU, not the gun.
Another is taking care of your gun. That's a PERSONAL thing.
- If you don't take the minimum basic care of your gun, any problems this may cause is YOUR fault, not the gun's.
- If you don't clean/lube your gun as you should, that's on YOU.
- If your gun, which has never given you problems before, starts giving you problems because you haven't used it since you packed it away in the trunk of your car two years ago, that's on YOU because lubes don't necessarily retain their proper lubrication characteristics indefinitely, repeated heat/cold/moisture cycles cause problems, and you buried the soft case under a varying load of tools and general cr*p.
- If you've run 5,000 rounds through your gun, and don't understand that sometimes springs need to be replaced after a lot of use, that's on YOU.
- If you carry your gun every day, but don't pull it out once in a while for general cleaning/lubing, that's on YOU. (Have you ever seen the amount of dust bunnies that can accumulate around the slide/ejection port alone?)
- If you abuse your weapon in any fashion, that's on YOU.
The fact that any other weapon may or may not have problems under similar circumstances does not necessarily reflect good or bad on THIS one.
WEAPON PROBLEMS:
Yep...sometimes it's really the weapon. Might be poorly designed. Might be a bad part. Might be a manufacturing/QA issue with this one pistol. Maybe the gun wasn't actually manufactured within the design tolerances specified for that particular model of weapon (a LOT of reliability issues with 1911s, in fact, can be attributed to this).
BACK TO "ZERO JAM":
SO...saying there's no such thing as a "zero jam" pistol is really a misnomer. Because there is also no such thing as a "zero jam" wheel gun, no such thing as "zero misfire" ammunition, no such thing as a "zero fault bolt action", etc.
Because there ARE no absolutes like this.
HOWEVER...there IS such a thing as "virtually zero jam", where "virtually" means "nearly", "almost", "for all practical purposes", "never had a problem ('yet', 'after X number of rounds')", etc.
The difference is an acknowledgement of the "practicality" inherent with the real, imperfect, world we live in.
My advice is not to get to hung up when people say things like "zero jam" as an absolute concept. Because common usage really means "virtually zero jam".
If a person has gone though a significant amount of time and ammunition to verify their weapon is reliable under whatever conditions (like some of the methods I discussed above), then as far as I'm concerned it's a "zero jam" pistol.
