The MML3 is the most effective missile launcher you can mount on an IS mech unless you're short of crit spaces or just want as many LRMs as possible. And that's before the massive BV rebate MMLs get (IIRC about 40% off).
Don't think there's rebate, especially when we consider that MMLs can fire only one type of missile. At least i would assume they'd get priced as if they were the more expensive pure LRM or SRM option.
BVs, using given MML values and using HMP's calculator for LRM and SRMs, though old it seems to get most things it can calculate perfectly.
http://www.heavymetalpro.com/bv_calc.htmResults:
MML-9 86, LRM-9 78, SRM-9 81.
MML-7 67, LRM-7 63, SRM-7 65.
MML-5 45, LRM-5 45, SRM-5 47.
MML-3 29, LRM-3 29, SRM-3 30.
Looking at this, MML-3 and MML-5 certainly seems to be efficient, but extremely marginally so. Bigger MMLs lose efficiency.
Also i can't help but wonder if any efficiency vanishes in practice due to MML bulk and weight though. While 3xMML-3 would be lighter than a LRM-10 (and 3 BV cheaper), they require triple the space. Advanced Inner Sphere 'Mechs tend to be very low on criticals.
On the other hand, 2xLRM-5 is equal cost to LRM-10 but lighter and easy to fit.
So what would balance them out? Do they need to be heavier, bulkier, what?
I tried to take the LRM launchers weight in mind when I planned out these weights. The UMMLs were rather tricky because Ultra ACs don't add a whole lot in weight for their work, and I based it on that.
I hadn't worked on BV yet, because I'm still haunted by doing it by hand when it was first introduced in the 90's, and I'm not sure how far I'd want to go with it.
Weight and heat don't usually impact weapon BV calculations at all, VSPLs being an exception. Well, MMLs too seem to be weird exception, so no entirely sure how their BV should be impacted.
Naively thinking, doubling LRM damage potential should boost their BV perhaps by 50% or even 100% (BV doesn't seem to care about average, only potential max, nor does it take into account potential failures like Ultra-ACs have). This wouldn't necessarily be an issue though in itself, as doubled-LRM counts would mean the launchers would be better LRM launchers with SRM capability thrown into mix.
I really don't know how to tweak MMLs to make their LRM side better or more attractive. If the weapon requires more weight or space, it makes standard launchers far more attractive option. If the weapon gets smaller and lighter, it becomes too good... or just even better "SRM launcher".
It did occur to me that MML-exclusive special ammo could be an option.
A couple of ways this could be done:
-LRM ammo that has some special feature at the expense of ammo per ton (half?), and can be only fired with MMLs. What this special feature would be is the question. Armor piercing LRM, combined Narc and Artemis LRM*, some kind of high-power LRM that has higher damage... Not sure about specific bonus. Just something exclusive to MML LRMs.
-"MML ammo", half amount per ton, has its own range brackets. Perhaps a mid-point between between SRMs and LRMs? With this, MMLs would be closer to emulating ATMs. But since SRMs are 2 damage and LRMs 1 damage, figuring out this "MML ammo" is difficult. Possibly 2 damage, 5 point clusters, range approximately 15 hexes, short minimum range.
(Quick note that ATM standard ammo is better than one might expect:
see this thread.)
-Or your double-LRM MMLs could be made into a special ammo option: Not compatible with other special munitions or standard LRM launchers**, half ammo per ton, LRM damage and range brackets, fires 2x rated amount of LRMs per shot. Figure a cluster or to-hit penalty may be required, to keep these from being dominating option and to create "BV sink" so to speak, as there's no rules for ammo types having different BVs.
*Clarification here because how i see it isn't simple: one benefit at time only. Narcs do allow for spotter-less indirect fire, while Artemis doesn't work with indirect fire at all. This kind of ammo would still be useful, especially for a force that uses Narcs and has MML using 'Mechs with Artemis IV. Also, Artemis stops functioning if just fired through ECM field, while Narc benefit works as long as the homing pod target isn't under ECM. Perhaps too marginal benefit for its complexity, and relies on Narc or Artemis presence.
**Annoyingly arbitrary but i figure otherwise this doesn't actually benefit MMLs really.