i never understand this issue people have with ACs....the guns are fine, it's the dynamics of how 'mechs use weapons that changed. ballistic weapons are are low-heat stopping power, so you can increase your sheer damage output without subtracting other weapons to make up the waste heat at the cost of being very heavy. energy weapons are utility weapons, they don't run out of ammo so you can use them to fire on bad odds without risking the weapon running dry at the cost of reducing how many other weapons you can safely fire at once. and missiles are specialist weapons that sacrifice in direct stopping power for the ability to do lots of things other weapons just can't like indirect fire and exotic munitions that can affect the battlefield instead of just attacking.
nothing about any of these changed, but freezers changed the entire dynamic of how 'mechs can fire. now light and medium 'mechs have far more capacity to simply pile on energy weapons and go to town without the heat slowing them down, heavies can mount far more firepower without sacrificing a lot of tons to the guns and Assaults....well.
Autocannons work just fine in my view. the specialty Ammo is also fine, and i never saw the drawbacks as pointless (i didn't LIKE them, but that's something else entirely)
LB-X ACs are a nice upgrade, and they also do a fine job.
Ultra Autocannons only suffer because of the "broken entirely" jamming rule and are otherwise nicely balance for what they bring to the table in my view, and Rotary AC's make for a nice substitute of you're afraid of the snake eyes.
what's wrong is just that technology has made them ineffective weapons for 'Mech in general because energy weapons have lost most of the weakness that Autocannons and other ballistics were supposed to cover, and that happens.