Just to explain (don't know if you know this or not) - most ships of this era wouldn't have that sort of gun fit for a couple of reasons:
- Difficulty aiming the guns - this pre-dates radar ranging, so seeing fall of shot is still an optical process. If you're close enough to be using your 5" surface guns you're probably shooting at it with the 6" guns too, which is going to make it very difficult on whoever's trying to work out which shells are hitting the target and which guns need to adjust their aim
Ammunition storage - any ship can only carry a finite amount of ammunition - yours will have to devote some space it could have used for 6" shells - which are generally more effective at damaging a given ship - for 5" shells. This could cause problems, if, for instance, your 6" shells run out and you're left with a bunch of 5" shells only.
AA - ships of this period would generally have a couple of AA guns, so it's a sensible substitution.
Lastly, I did notice one more thing - generally, warships are fighting a constant battle against topweight - weight that's carried high up, compromising the ship's stability - some inter-War Japanese ships ended up capsizing because the IJN were a little overzealous in their equipment fit and both the US Atlanta
-class and the British Dido
-class ended up having to change their equipment and armament fits during the war (generally as more radars - more topweight - were added) to address stability issues.
One way to prevent this problem developing on your Philadelphia
would be to switch your heavier 5" guns and your lighter 20mm Oerlikons around, getting the most weight that you can down low.
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