On a large enough map a Fire Moth with a cERLL could kill a regiment without taking a shot in return. The question is; where's the artillery? Air support? Forward infiltrating hovertanks with carried infantry?
Thats the problem with big gun battletech in a nutshell. If there is a mech regiment and a single dasher on a sufficiently large map, the artillery, air support, forward ops bases arnt on the board and dont exist. Also, there was hours upon hours spent gathering the regiment wherever it is. And, there is no falling back from the harrowing dasher--leaving the board counts as destroyed. Also the regiment doesnt have any objective, the base game is 'play until one side is totally destroyed.'
You CAN put all those things in, but the base game doesnt support all that... Battleforce starts to include that stuff, and people say battleforce is a great game (I have barely dabbled with it).
Rolling maps is a bad mechanic for the game of battletech we have. The range/rules abstraction doesnt support large, especially infinite maps. The weapons and ranges are balanced on the idea that you can box in a faster unit, otherwise something like a grendel prime with its 7 jump jets and ER large laser just couldn't die versus something slower with 24 range or less.
As for why mechs dont carry 100 shots, despite the apt description of ww2 comparable combat? Well, again its scale. Its notable that as soon as you pull out battletech to the next level up, alpha strike, well 10 shots = infinite shots with no ammo at the alpha strike, 12ish v 12ish scale level. At the battletech scale though, for the 4v4ish gameplay 10 rounds for an AC10 is enough ammo that you can run out, and the decision between 10 or 20 shots is meaningful. If running out of ammo is something wanted in the game, it needs to be a number you can reasonably fire in the game provided. Like, the joke with mgun ammo is the hilarious shot count, but like you mention in ww2 style combat, a machine gun that can fire small bursts 200 times is about right with how much suppression fire those tank mounted mguns do. If battletech was played 12v12 or larger with 50 turn games, like mentioned everyone is forced to take only energy weapons because the game just doesnt support that scale by design.
Now, the IRL thing to do would be to just, carry ammo backpack cargo with the mechs, or with a truck or something just off-screen. Like foot soldiers carry extra equipment, its silly to think that a mech that marches for 600 km does so with 0 extra support or ammo. In a strategic sense, the enforcer or trebuchet would have sling loaded or truck toted ammo, and it could replenish itself when it had time, by just dipping back for a little bit behind cover as long as the flank was secure. Bradley IFVs have extra missiles, and someone has to get out and manually reload them, and it takes time/not something to do while fighting, but its doable, so the same would be true for a Trebuchet, which is probably why the scaled out alpha strike/battleforce games dont care about your ammo as long as you have 10 shots, and they penalize damage only with less then 10 shots.
The entire point, is that small weapons and close maps with hard borders are required for the game as presented to function, and the game isnt really designed to be played much longer then after 15 turns of shooting as a regular occurrence. There are other games that are designed to handle larger scales of battletech for that, and one of the first things they do is remove ammo considerations.
