I know for sure that the round fired by the Glock 17 (their standard 9mm service pistol) fires the same round as the 9mm Beretta, the M9 (known in the civilian world as the 92… I know quite a bit about guns, and I have NEVER heard of a 9mm round with a “square back”. In fact, I’ve never heard of ANY centerfire cartridge with a squared-off rim. A square rim would screw up how the round would sit in the magazine, and how it would feed into the chamber. The only “square” “cartridges” I’ve ever heard of came with caseless rounds, and weapons using caseless ammunition is so complex that it has yet to be fielded in any form.Now, when I read that, I knew immediately what the writer meant. On the back of the shell casing is the primer. When it strikes the primer, the firing pin leaves a mark. Glocks use a striker with a rectangular (or "square") cross-section, unlike pretty much every other pistol on the planet, which all use a round firing pin.
I could walk onto a commercial firing range littered knee-deep in spent 9mm shell casings, pick one up, and immediately tell if it was fired from a Glock by the rectangular firing pin imprint on the primer (or "square back" in non-technicalese).
Let's be careful to stick to debunking the factoids we're sure of, okay?
I'm no Oleg Volk, and I apologize for the crappy picture quality, but on the left is a round of .357SIG brass from a Glock 33, and on the right is a .40S&W shell casing from a Beretta 96. Note the squarish outline to the primer strike on the left-hand shell casing as opposed to the round dimple on the right.