I think the problem is that the missile is considered the weapon instead of its launcher. That said BT OS Missile Launchers are terribly inefficient (because they are counted based on the launcher) except for the rocket launchers, because of that I think they should be removed except for Battle Armor (and only then for reenabling jump capability after a missile ambush). At least the Fusillade and Elemental have enough sense to give you two shots.
Could these be improved? Maybe, but it would have to be half the mass of the launcher with no additional penalty for the ammo.
There is an OS Energy Weapon, the Dragonsbane Pulse Laser, which is fluffed as being capable of seriously harming an elemental in comparison to a regular SRM before its optics burn out (and was featured in one of the better IC fics in LosTech). I think a Heavy OS chemical laser could be a viable weapon on Battlemechs but it would have to do a lot of damage (either 20 like a Cap+HPPC, 16 for a HLL, or 12 for a Blazer) and have some disastrous side effects if hit for it to be viable. It would also need to be short ranged (maybe 4/8/12), compact (no more than 2), a little heavy (at least 3-4 tons), and hot when popped though to make it viable (and nonviable or explodey for vehicles).
For ballistics I think the gun being the heavy part is the problem, now you could have something like a Recoilless Rifle but those are not treated as 'ballistic' as I recall. Arguably land mines, and certainly Vehicular Grenade Launchers plus the (A,B,M)-pods are OS ballistic weapons.