8" naval gun
The 8" naval gun (203mm) was defined, by the Washington Naval Treaty by the point of reference for a heavy cruiser main battery. Historically, it has been an exceptionally accurate weapon. The naval weapon is longer-barreled than a howitzer, and the 8" howitzer was long considered the most accurate artillery piece in the U.S. Army.
While it is not in active use in any navy, it is often considered the heaviest practical caliber for naval gunfire support. Advocates of reactivating battleships of the Iowa-class for NGFS argue the 16"-50 caliber MK 7 naval gun is a superb weapon, but it is less accurate, and its larger explosive charge prevent it being used as close to friendly troops as an 8" can be.
U.S. cruisers of the Des Moines-class, which had high rates of fire, were considered extremely effective for NGFS in the Vietnam War, probably more so than the Iowas. In some tactical scenarios, they could actually deliver more explosive to a target area than could 16" guns.
It has been accepted that 5" naval guns are too light for many NGFS missions, especially in the presence of shore defenses when the guns are on billion-dollar destroyers. The major trend has been to move to 155mm guns for NGFS, often rocket-assisted for greater range. Nevertheless, there have been a number of experiments with lightweight 8" guns optimized for NGFS, rather than cruiser anti-surface warfare, and there are substantial data and advocates of 8" over 155mm.
Guided shells in 155mm more than compensate for pure ballistic accuracy of the gun, but the greater shell weight may be advantageous for area coverage.