STANDARDIZATION MARKINGS and MEANINGS
In August 1959, the NATO Department of Military Standardization published the NATO identification marking. The NATO identification mark is a cross within a circle. All types of ammunition standardized by NATO and adhering to the terms of the Standardization Agreement (STANAG) bears this mark, indicating that the cartridge in question is interoperable with others so marked. However, it cannot be stressed enough that this marking alone does not guarantee interchangeability. That is to say, a random selection of 7.62mm NATO cartridges from different manufacturers bearing the cross-within-circle marking will likely display equally random ballistic performance. Only the inclusion of an additional marking on the cartridge packaging in the form of a maltese cross or four leafed clover indicates that the ammunition is expected to provide identical ballistic performance. If the clover symbol is framed, this is an indication that the accessories contained within the packaging such as clips or links also correspond to designs approved by the STANAG.

Based on the existence of these two distinct markings, it is evident that there are at least two types of NATO 7.62mm military ammunition. The first, bearing only the cross-in-circle marking, will function in all 7.62mm rifles and machineguns fielded by NATO armies, but may or may not provide the expected ballistic performance. The second, bearing both markings, will not only function in all arms, but will provide the ballistic performance prescribed in the STANAG (a 147 +/- 3 grain bullet with a muzzle velocity of 2,750 +/- 50 feet per second). The picture is muddied further for the US consumer of surplus 7.62mm NATO ammunition by the fact that many nations produced three types of military 7.62x51mm cartridges:

Those with cross-in-circle markings on the cartridges and cloverleaf markings on the packaging;
Those with only cross-in-circle markings on the cartridges
Those with NATO standardization markings
Consequently it is all but impossible for a buyer on the surplus market to know which standard, if any, the 7.62x51mm ammunition he or she has purchased adheres to.

.....NATO specification rounds (those marked with the NATO "cross in circle" mark) may have thicker cases and are thus able to withstand stretching better when fired in loose chambered military weapons (done for reliability purposes).