I took it upon my bored self, after I rode my bike to the post office for 6 miles one way (only to return the other 6 miles empty handed because they mis-delivered the parcel which didn't have enough postage on it) only to do an experiment and measurement of projectile/case in both metric and standard measurements with digital calipers with a handy mm/inch switch button. I got to thinking after my other post about being able to fire a 7.62 NATO out of an AK-47 that there really are a lot of .30 cal rounds out there in all their variants. The two pictures are taken from different angels, one to see the difference in size by height, and the other is to read the measurements of them I have handwritten. The very top hand written numbers designate the round itself. The numbers hand written below the underline are written in millimeters, and are the actual measurements I made of each individual cartridge at the top of the brass (on the AK 7.62x39 and the 7.62x54R I measured at the top of the red finish). Then for case length I lined the caliper up on the bottom as best to center of the primer as I could, flatly contacting the bottom of the shell to the open caliper, and measured to the top of the case. I did not do any collective overall length measurements. If anyone is interested, I could for any of them. The .30 Carb was hand re-loaded, The 7.62x39 is Wolf manufactured, the .30-30 is old but isn't re-loaded (unknown manufacture), .308 Win is Federal factory ammo, 7.62x54R is Albanian surplus, .303 Brit is Federal factory ammo, the .30-06 is hand re-loaded, and the .300 WinMag is also Federal factory ammo. Unfortunately I do not have any 7.62 NATO to put up against the .308 Win for actual measurements. I found it interesting, by the common measurements we know these by, they just didn't seem to match up everywhere. Thats where I don't know why. Buuuuuut, I won't lose any sleep over it tonight, because I know what shoots out of what. The other interesting thing was they were all of nominal .30 cal on the standard measurements. But I'm not a machine, and the digital calipers told me what they did, so I'm sure there is room for human error as well as mechanical. Anyways, the frowny face on the .303 I accidentally shot it all so I bought more today. None of the ammo pictured, to my knowledge, is inter-changible and able to be shot out of a different gun unlike a .308Win or 7.62 Nato.