Variable Speed Pulse Lasers are also extremely good so I'm just in the position of having to disagree extremely hard with Cray's opinion on both.
If VSPMLs had been 2 tons then they'd be awesome and I wouldn't complain.
I just had to write-up a Spider variant with 2 medium VSPs for one of the ilClan Recognition Guides. I had wanted to build the Spider around a single Clan ER large laser and 10/15/10 movement. Instead, the design sacrificed 8 tons for 2 VSP medium lasers weapons, dragging its useful range down to 5 hexes while its speed was down to 8/12/8. That was fine in 3025 but it's suicide for a 'Mech expected to kill Clan units. The VSPML is not competitive with Clan medium pulse lasers, let alone the many heavier Clan weapons that can easily evaporate a light 'Mech dancing around in those weapons' short ranges.
Given that I was stuck with 2 VSPMLs, it wasn't hard to consider all the alternative configurations possible for 8 tons: 8 medium lasers, 4 medium pulse lasers, 4 medium X-pulse lasers, 4-6 ER medium lasers and heat sinks...all of them delivering more average damage than 2 VSPMLs. (And all just as suicidal for the Spider. Light 'Mechs survive by keeping target numbers high, which means long range and high speed.)
The VSPML's -3 to hit and 9 damage at 0 to 2 hexes is nice, while the -2 and 7pts to 5 hexes is only okay. And it's hindered by the 4-ton weight. You can do better with almost any combination of 4 tons of other medium lasers.
ATMs of equivalent weight will do less damage at long range, equivalent damage at medium range, and massively more damage at short range.
For an equal tonnage of tubes, the damage isn't massively greater but more like 20% greater just over a range of 1-3 hexes. And ATMs need 3 types of ammo to get that mild damage edge while the LRM doesn't.
An ATM 6 and an LRM 15 are the same weight; the ATM generates less heat (4 vs 5) and has more ammo per ton (10 vs 8). ER ammo has better range than an LRM by a substantial margin but ignoring that for the comparison a single ATM 6 will average 5 damage at long range to the LRM's 9; standard ammo averages 10 damage at medium range to the LRM's still 9; HE ammo averages 15 damage to the LRM's 9.
In terms of hit numbers the math is more complex thanks to different ammo types than I'm willing to do on my break
It does take a spreadsheet.
I had a spreadsheet cooked up for assessing different weapons at different target numbers, but there was an error in the cluster hits averaging page that I never got around to fixing.
Like with the medium lasers, the target numbers radically affect damage. 2d6 rolls get very different results with the +/-2 change of range bands. So...
ATMs do have an advantage over Clan LRMs at 1-3 hexes, though it's not "massive" for an equal weight of launcher. Using your ATM 6 vs LRM 15, you've got the same target numbers at ranges 1-3 but 18 base damage versus 15. ATM HE ammo promptly becomes uncompetitive with Clan LRMs at 4+ hexes.
ATMs ER ammo has good range bands, which help it. As I recall, the range bands are good enough that standard ATM ammo isn't very useful at all - you can get by with short and ER ammo. ER ammo's to-hit bonus is good enough that it is comparable to LRMs at 8-9 hexes and 15-18 hexes. However, having less than half the base punch (6 vs 15) means that when range bands are equal (5 to 7, 9 to 14) LRMs are better.
but it very broadly comes down to the ATM having the luxury of choosing good ranges and moderately lower damage, or significantly improved damage.
That's not a luxury, it's an unnecessary complication that the Clan LRM avoids.
The ammo needs are definitely a concern for smaller launchers, but as soon as you have a many as 12 tubes (usually two 6s) you'll have access to any ammo you want and tonnage parity with an equivalent LRM.
ATMs won't have mines, ATMs won't have smoke, ATMs won't have infernos, but they will need 3 types of ammo to slightly outperform one type of LRM ammo. That's not an advantage for ATMs.
EDIT: these numbers get much better for the ATM when streak capability is included, and the ability to indirect, and defeating non-Angel ECM, and being special ammo, and, and, and
The damage advantage is, as noted above, modest, and you need 3 types of ATM ammo to replace 1 LRM ammo type. iATMs do have some specialty ammo (magnetic pulse warheads, infernos) which is nice for individual combat but doesn't offer the campaign utility options of mines or smoke.
The indirect feature is a very nice patch to one of the ATM's greatest flaws, but tends to be wasted on Clans. You need spotters and cooperative engagements, which Clan warriors tend to avoid. If you do get spotters helping the shots then, yes, iATMs get close to parity with Clan LRMs.
The Streak feature is nice, but it's a heat and ammo saver, not a game changer. Clan energy weapons are weight competitive with iATMs, do full damage on every hit, and avoid ammo usage, too.
