Reynod :
Yes.
The NATO round is 51mm long ... not quite as powerful as the .30-06 ...
Both not correct.
The "X51" part of the designation does not designate the length of the round. The NATO round is about 3" (75 - 80 mm) long. And it is just as powerful as the standard .30-06.
From wikipedia:
"The cartridge itself offers similar ballistic performance in most firearms to the .30-06 Springfield that it replaced in U.S. service. Though shorter, standard loadings fire similar bullet weights at similar velocities. Modern propellants allowed the same velocity from a case with less capacity. The smaller case requires less brass and yields a shorter cartridge. This shorter cartridge allows a reduction in the size of the firearms that chamber it."
Both cartridges result in 158 grain slugs (standard military loads) at a muzzle velocity of about 2800 fps. The .30-06 has more potential when handloading because of the larger cartridge case.
Personally for a sniper round, I'd prefer .338 Lapua.