Yes, there are differences between the commercial .308 Winchester and the 7.62x51mm NATO round. The military chamber dimensions differ slightly from the SAAMI chamber.
For all practical purposes, the cartridges are interchangeable. The commercial round followed the NATO standardization of the rifle caliber round, of course.
1. The 7.62 NATO round is nominally available as a 149 grain FMJ ball, a tracer, and an armour piercing round. These two are approximately 173 g. All can vary slightly from one country of manufacture to another. With the NATO standardization (headstamped with a small circle superimposed with crosshairs) these cartridges can be used interchangeably in 7.62 NATO standard weapons.
2. Various member nations of NATO have also developed specialized 7.62 NATO cartridges, especially for competitive target shooting and sniper rifles. These are optimized for various weapons platforms (US M14m Canadian C1, British L1A1, etc). A few specialty ammunitions have also been developed, including multi-ball rounds for use in mini guns.
3. Various non-NATO countries (Chile, Peru, Argentina, Brazil, Thailand, India, Taiwan, Australia, etc) have also produced 7.62x51 ammunition. These are NOT NATO standard becasuse they don't have the NATO certification standards or inspections, but are nominally safe to fire in all 7.62 NATO weapons.
4. .308 Winchester commercial ammo MAY be loaded to higher chamber pressures than military ammunition. They may also be unsuited for use in some NATO weapons. Examples include: 180 grain and 220 grain softpoint ammunition is unsuited for use in the M14/M1A can damage the action from excess port pressure, or in a G3/HK91 because of excess recoil velocity.
What does this mean to you, the shooter of surplus ammunition? Not much except that the military 7.62 cases are usually heavier and thicker compared to commercial .308 W, and reloading require reducing the powder charge for gas operated actions.
