FinFin .. folks are talking apples and oranges here ... The circle with the cross, "The Nato Mark" just means the ammo, as originaly loaded, meets Nato specs as to bullet weight , velocity, and pressure for one of the standard offical Nato loadings, and will shoot within Nato specs using the sights on any official Nato weapons regardless of what Nato country made the ammo or the weapon.
Doesn't mean it is maxed out in pressure by any one standard or another, doesn't mean it is .223 or 556 or 9 mm or 7.62 x51 or .308, which all have , if they meet Nato specs, the mark that says they meet Nato specs, the circle with a cross. The specs are slighly different for tracer or AP or Ball ammo in all Nato offical calibers, because different types of bullets have different weights , need diffrent types of powder loads etc but if they meet Nato specs for Nato ammo they get the Nato Mark.
Just talking .223 /5.56 here for a minute , one can assume that Nato ammo is safe and within the proper milspec to fire out of any Nato approved weapon in that caliber. Lots of different .223 out there can't get the Nato mark for a variety of reasons , uses a expanding bullet outlawed by various Law of Land Warfare treaties, lot of .223 varmit ammo uses powders that have pressure peak at the wrong place in the barrel to be within spec on Nato semi or full autos, great for .223 bolt action varmit shooting though, maybe uses a VLD 75 grain longrange competion bullet that won't fit in a magazine. Doesn't matter why, not in milspec, doesn't get Nato mark.
Nato ammo has a spec sheet and a "M" designation, regardless of caliber.. 120mm Tank rounds that meet Nato standards have the little Nato mark.
That all said, milspec brass is a little heavier in weight and thickness because Nato said that is how they want it in the contract.. I assume that is as an addition safety measure, or maybe so the brass will still be shootable after being dropped out of a helicopter, or maybe to slow cookoffs in a burning vehicle... not that it really matters.. you meet the spec or you lose the contract and don't get the Nato mark.
And all semi and full auto's have some restrictions on what powders and loads will work in them , not all as strict as the requiremets for the Garand, some just need enough pressure and power to function the action, but ammo that won't function the action in a semi , will work fine in a bolt gun. Ammo that could wreck a semi can be safe in a bolt gun.
Garand is a good example though, you can use bullets up to 170 grain with the right powders and not hurt the rifle .. but if the Ammo says it is M2 Ball 30-06 it is a 150 grain spire point flat base lead cored cupronickel jacketed bullet being pushed by enough 4895 powder to make the bullet go 2740 fps. There are IIRC some very limited authorized substitutions to that spec. The spec ain't near max allowable pressures for the cartridge itself, it is delibertly downloaded from earlier M1 ammo spec ( not to be confused with M1 rifles of various types) because of the cost of having rifle ranges that are big enough to shoot M1 spec ammo safely, and M2 30-06 made to exact US specs will never have a Nato spec mark because 30-06 is not a Nato approved caliber, and thus Nato has no spec for 30-06 to meet to get the mark.