Fuchs:

Carbine/rifle firepower quality is being overestimated and it's still the machine gunners and snipers that do 80% of the job (~Pareto) - just as they did in the age of bolt-action carbines.
I define sniper in the context of an infantry squad or platoon as quite the same as designated marksman.
* an emphasis on camouflage, concealment, cover and deception to the point that he becomes quite invisible even while shooting (including muzzle flash and dust cloud concerns)
* aimed single shots of unusually good accuracy

To me the difference between a sniper and a designated marksman is
* DM is organic to infantry small unit, sniper at most attached to it
* sniper is more extreme; more specialised weapon, more fieldcraft training
* sniper is trained as forward observer and usually better at ranging
* sniper has greater freedom of movement relative to small infantry unit.
* sniper has his own fire discipline
* sniper isn't meant to participate in assaults

So in the context of this discussion they're practically the same and I become occasionally lazy enough to not differentiate between them.

M16A1 LMG.
WW II most efficent Finnish sniper Simo Häyhä used rifle that was used by regular infantry with open sights. No optics, no tuning, just marksmanship and discipline. He killed approx the same number of Soviets with Finnish SMG. Single shot rifle plus SMG. If you have red interviews with German WW II snipers then shooting distance 500m was quite rare. Today US military has rifles with Trijicon sights. Hyh would envy. It all depends on training like Ken says.

M16A1 LMG. Just two thoughts. Those rifles didn't have free float barrels and barrels for accurate shooting at longer distances.

I have been thinking for some time why we need those small unit level marksmen at all, when there is possibility that machine gunners can hit the same target with 10th round? ... or why we need machine gunners if we have marksmen in small units? Machine gunners were in the beginning compensating low shooting speeds of regular infantry with bolt action rifles. That fire power aspect. Second, due to the bipods and tripods machine guns are more stable for long range shooting. Today you can attatch those shooting aides to your rifle rails. And here we are talking about marksman's kind of machinegun IAR

Like many here have mentioned here there is ballistics aspect. 7,62x51 flies further and penetrates better than 5,56x45. That's why MG's are still good asset for regular infantry with 5,56 ammo. Will this change with 7,62 IAR?