On the other hand we don't hear about many naval engagements being active for more than 30 minutes without having enough time in between to reload the magazines from stocks. Regardless it is a very minor difference, even increasing the total to 120 (2 hours of direct combat) only increases the total mass of the NACs by 324 tons - and I actually miscalculated the mass of the ammo in the original calculation, using 2.5 tons per round rather than the .4 tons per round it actually is. The new calculation is that 5 quad NL55 bays is equal to 2.95 triplet NAC/20 bays (up from 2.92; and this new number is with 120 rounds along with everything else). Is this a shorter leash than lasers? Yes, but not significantly so, depending on the amount of stores or if there is a fleet train backing up the combat ships - which it honestly sounds like there is as the Republic is purposely putting resources towards anti-pocket warship units.
In pretty much every case, the exception being capital missiles, ammunition is a minimal investment into capital weapons, we have to get to more than 317 rounds per NAC to finally reach the tonnage of a single NL55 - and this is more than 5 hours worth of active combat. If logistics are a factor, NAC or NGauss ammunition is such a minor investment as to not being worth using as a limiting factor - warships aren't like BattleMechs, where ammo is a serious concern due to the much more limited space issues.