From Europe, one can get the impression that under the U.S. patent system >everything< may happen.
Most nations require some intellectual level of the invention in the sense that things obvious to a professional cannot be patented. MEN in Germany once tried to patent making a more powerful cartridge by lengthening and increasing the diameter of the cartridge. (I am not joking!) They failed. As far as I can tell, dimensions as such cannot be patented because of lack of any level of invention as long as we are talking of an ordinary cartridge layout.
The case RonMerchant mentioned (US patent 841861 of 1907, German 204660 of 1904, both assigned to DWM) is something very different: a certain bullet shape for aerodynamic efficiency, very different from what was common at the time. (That the idea of this shape was effectively stolen from the French is another matter. But the French decided to say nothing; classification as an end in itself - sounds familiar.)
Last not least, all patents expire. Cartridges like 7.62x51 are much too old for being under patent protection. That makes the Russian whining about former allies who produce “unlicensed” AKM clones (a design at least 60 years old) so funny.
Trademarks are a very different matter, of course. For many years, Ford could not call its car Mustang in Germany, because Krupp had been in the automobile business (actually trucks) and had obtained a trademark on this name. When the French brand Simca renamed itself Talbot, they had to settle with a German railway rolling stock maker of the same name (Waggonfabrik Talbot, Aachen). Trademarks do not automatically expire after a given number of years like patents.