"Grunt" can be used pretty loosely. Recon Marines are not special forces, there somewhere in between special forces and grunts. They can use any rifle they request, if a Recon Marine wants a sniper rifle, he gets it, if he wants an AK-47, he gets it.
Recon Marines falls in the S.O.C (Special Operational Forces) branch, but their role is specialized the same way a sniper battalion is.
Grunt has wide meanings, not arguing that, but taking a look on the perk description and even the image the game portraits, I think the "grunt" mentionated is the regular soldier founded in manny armies around the world.
That's why I said "no sniper rifles", usually snipers receive more specialized training than a regular trooper.
This doesn't mean a trooper has to be assigned for "kitchen duty", you have heavy troppers and regular soldiers are provided with an variety of weaponry, like carbines, grenades, C-4, grenade launchers and such.
Even in NV a trooper armed with All-American/Marksman Carbine, Combat Armor reinforced II, some frag grenades and a side-pistol is not what I call "weak" in combat.