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The “Iowa”-class gun barrels had a rated life of about 300 rounds (depending on the powder load and other factors).

That was when they came into service during WWII. Following the war advancements in propellant (during Korea), preservation chemicals (like "Swedish Additive", which was a titanium dioxide and wax mixture, during New Jersey's Vietnam reactivation) and other measures (wrapping the propellant bags in a plastic film during the 1980s reactivation) resulted in something like a 4x barrel liner life increase for the AP "heavy" shell and an 8x increase for the standard HE shell. IIRC when they left service the estimates were that the barrels could handle over 1500 shells before needing replacement. Each. So over 13,500 rounds per ship and 54,000 rounds for the entire class, from an assumed point where they were completely re-barreled (note: of the class only NJ was re-barreled during their 1980s reactivation, receiving only a single new barrel that was considered to be worn out when she left service in 1969)

I don't recall whether tests were conducted or not with the 13" sub-caliber cluster-munitions sabot round before they left service, and the 11" sub-caliber (GPS-guided with a notional 100 mile range) was still in the concept phase at DARPA, but it would be assumed that both of those would be fired using reduced charges (something the Iowa was playing around with at the time of the turret explosion) that would extend barrel life even further.

But enough about that. The Iowa's two primary missions went away with the mass adoption of VLS (including retrofitting of most of the Spruance-class DDs) that took away their cruise-missile-shooting supremacy, and the introduction of GPS-guided bombs that took away their all-weather shore-bombardment supremacy. The simple fact is that a wing of B-52s loaded with JDAMS is MUCH more preferable to the Iowas in just about every respect.

This inferiority in the Iowas' capability has been recently augmented with the Ohio-class SSGN conversions, which are - functionally - the arsenal ship through other means.