There are two reasons people want to replace AC-5s with AC-2s. Neither applies in reverse. AC-2s are lighter than AC-5s so you gain tonnage rather than having to find something to sacrifice. Unless you're starting with that one Clint variant with two AC-2s on one arm or the Daiboku/Mauler you can't go the other way.
And secondly, AC-5s are inefficient and have no niche until the development of precision ammo while AC-2s are inefficient but have an important niche as anti-air and to a lesser extent anti-vehicle weapons.
The Bane conversion runs into the Clan Ballistic problem. They gain less from clantech than missiles and energy weapons and the Ultra-5 runs straight into the LRM's niche where the Ultra-2 could distinguish itself on range. That's a problem for Inner Sphere Ultra-5s as well, but it's even worse for the Clans.
3/5 was acceptable for primitive mechs and some of the early modern designs made the mistake of upgrading incorrectly, but nobody fields assault speed heavies until after New Dallas except some odd customs like the Bounty Hunter's Marauder. They clearly do not work well in-universe for military operations as established by several centuries of mech design. The reintroduction of failed designs may have made Doylist sense as a way to sell miniatures, but it never made Watsonian sense and was just another stupid thing on the towering pile of stupid that was the late Jihad, Republic of the Sphere, and Dark Ages.
Marching speed matters when your mechs don't appear ex nihilo on the field of battle. Assault mechs and urban defense tanks can get away with being slow and specialized and the tanks do 4/6 on roads which most inhabited planets presumably have. Something which's purpose is to keep its AA umbrella over Centurions and Enforcers and Blackjacks, though, can not afford to be slower than the mechs it's supposed to escort.
Worse, you're throwing away the only virtue of the Jagermech: that it can cover allies from fighter attacks 17% farther away than a Rifleman and isn't outranged by some of the helicopters it's supposed to defend against.
There is also the sheer range advantage of the AC/2 in 3025, and to a lesser extent in later periods. It is wonderful for harassing the opponent. A couple AC/2s usually aren't worth detaching elements to hunt down and kill the little plinker, yet its like an Annoyatron.
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All reasonable points. The 3/5/0
JagerMech, as a defensive platform (low speed), is not a world-beater, but again, this is just an experiment. The Partisan tank is probably better.
I disagree that the AC/5 does not have its uses on ‘Mechs, though. Only Gauss weapons have a more efficient damage/heat*. AC/5 is also modestly long ranged. The Clan equivalent, UAC/5, fires as far as LRMs.
Considering AoW or 3rd Succession War, the AC/5 compares with two IntroTech weapons, the standard medium laser, and the standard PPC.
Vs ML: Weapon + HS + ammo, 1 AC/5 vs 1 ML, the AC masses 6 more tons, requires 2 more crits; 2 AC/5s can share ammo, so 11 tons, 3 more crits, etc. In game play AC ammo can explode, ACs have a minimum range, but they also have double the range.
Take home, the Autocannon/5 trades a little size and significant mass to double the range.
Vs PPC: Weapon + HS + ammo, 1 AC/5 vs 1 PPC, the AC masses 7 tons less, and takes up 7 fewer crits. Again sharing ammo; 2 AC/5s vs 1 PPC, the ACs mass 2 tons more, but take up 6 fewer criticals; 2 AC/5s vs 2 PPCs mass 15 tons less, and takes up 15 fewer crits, etc. In game play, AC/5s do 1/2 damage (so 2 AC/5s vs 1 PPC is equal damage) and have explosive ammo, but have the same range and minimum.
Take home, the Autocannon/5 is a denser weapon per equal damage, but individually AC/5s are lighter and smaller.
This analysis ignores the engine heat sinks. When building a light, the free engine HSs really skew the ‘Mech towards Lasers. In fact, looking at 3025 ‘Mechs generally, their loadouts mirror the mass-space trends above: light ‘Mechs take lasers and use speed to negate range disadvantage, medium ‘Mechs take an AC/5 with limited mass but switch to PPCs if they have enough mass to increase damage, while (good) heavy/assault ‘Mechs use PPCs (and HSs + MLs are so small and light, you can mount them last to make the numbers add up).
Do I love ACs generally? Not really, I feel lasers are better weapons for a number of reasons (engine heat sinks, no ammo, no min range, ‘Mech heat scale, others?)
(A short aside: one of several IRL questions, waved away for game play balance is Gauss weaponry damage/heat. In real life, if a laser’s waste heat comes from the power feeds, how can a Gauss Rifle require charging a massive, explosive capacitor, but not also generate waste heat? I don’t remember, do CVs require amplifiers to use Gauss Rifles?).
I decided to not even discuss in depth how heat sinks take up too much space in assault ‘Mechs and slow heavies, where flashbulb ‘Mechs will suffer, but adding an AC or two remedies that stress. Again, do I love ACs? No, but I recognize their uses.
*ignoring zero-heat weapons and attacks
