Apologies if this has been discussed, but is there something going on with big guns, their accuracy, and their penetration values?
Boot up the "There Can Only Be One" NA mission.
No matter what guns I use -- even with 20" guns with 214in of deck penetration (at 20,000m) I mostly get partial pens. The enemy, on the other hand, can seemingly cut through my 20in deck, 15in fore, 14.5 aft with little to no issue. Modern armor II, so add 160% to those values. Oh, and maxed-out citadel with nearly maxed-out inner deck armor, all three levels. Enemy still scoring thousands of damage points with single 17-19" hits. Nonsensical.
Lastly the enemy is hyper-accurate, hitting far more often than they miss.
I've never died so many times (and I still have yet to beat it) in this game over a year+ of playing.