Ammunition designators are a bit confusing. In the U.S. we have the .30-06 but in Europe it is the 7.62 x 63mm. The metric designations make a difference since it is the bore diameter by the case length, in millimeters of course. Hence, NATO uses the 5.56 x 45mm for the .223 Remington and 7.62 x 51mm for the military version of the .308 Winchester. However, military and civilian loadings are not necessarily compatible. All ammo can be shot out of military rifles, but the reverse is not true.
Freebore is the distance the chambered round’s bullet is from the rifling before firing. It is the bullet’s bearing surface, the parallel sides of the bullet, and the distance is greater in a military rifle than civilian rifles. Generally, civilians shorten the free bore to improve accuracy. But military guns need to be reliable and only accurate enough. Military ammo is loaded hotter to compensate for the longer freebore and is dangerous to shoot in a civilian gun unless the chambering is altered to military specifications. Military 5.56 NATO ammo in your .223 Remington Model 70 varmint gun will produce higher pressures that can be dangerous resulting in a broken gun and a broken shooter.
Battle Rifle Cartridge
Most studies of combat have shown that the vast majority of engagements are at 350 yards or less. Full power rifle rounds are not necessary for the infantryman and a lighter round allows more to be carried for the same weight. Enter intermediate round. The first was the 7.9 x 33mm Kurz developed by the Germans during World War II. Kurz is German for “short”. Hence the .380 ACP is also known as the 9mm Kurz. In 1943 the Soviet Union introduced the most famous or infamous cartridge in world history, the 7.62 x 39mm round that wasn’t used in numbers until the post war SKS carbine was issued up to the AK-47, burned into the 20th century psyche as the arm of terrorism and insurgents.
5.56 NATO Standard
In Vietnam the US followed what was a US Air Force initiative and adopted the M-16 in 5.56 x 45mm and what became the 5.56 NATO standard for the western world. To keep up, the Soviet Union in or about 1974 introduced the current Russian caliber of 5.45 x 39mm used today. The US civilian designation is the .220 Russian. For general purpose machineguns, designated marksmen and snipers the full power ammo was retained with the 7.62 x 51mm used by NATO and Asiatic allies of the US. Those countries under formerly Soviet influence still use the rimmed Russian 7.62 X 54R round.
In Vietnam a common rumor was that the Russian 12.7mm heavy machinegun was a .51 caliber and could not be used in US guns. In fact, the rumor said that the US .50 Brown Machinegun would chamber is the Russian .50 caliber. Not true. That actual reason was headspace. Headspace is due to the space between the head of the cartridge and the breech. In rimless rounds they must headspace on the mouth of the cartridge like pistol rounds like the 9mm Luger and the .45ACP. In rifles headspace is due to the tapered angle where the case reduces its diameter to the bullet diameter. That angle acts to fix the cartridge in the chamber. Well, as can be seen there is a difference of 12mm between the 7.62 x 63mm length of .30-06 and the 7.62 x 51mm NATO. Same is with the .50 Browning Machinegun round and its Russian equivalent. The US round is the 12.7 x 99mm whereas the Russian is the 12.7 x 108mm, a difference of 9mm or over a third of an inch. Neither round could head space in the other guys guns.