Yep. Definitely need hard point restrictions.
I think an ideal solution would be the following:
Energy Weapons
Any configuration which has a Small Laser or Small Pulse Laser in its default configuration has a Small Energy Hardpoint. A Small Energy Hardpoint can mount any energy weapon that fits into 1 space.
Any configuration which has a Medium Laser or Medium Pulse Laser in its default configuration has a Medium Energy Hardpoint. A Medium Energy Hardpoint can mount any energy weapon that fits into 2 spaces.
Any configuration which has a Large Laser, ER Large Laser, Large Pulse Laser, PPC, or ER PPC in its default configuration has a Large Energy Hardpoint. A Large Energy Hardpoint can mount any energy weapon.
Ballistic Weapons
Any configuration which has a Machinegun in its default configuration has a Small Ballistic Hardpoint. A Small Ballistic Hardpoint can mount any ballistic weapon that fits into 3 spaces.
Any configuration which has an AC/2, AC/5, or Ultra AC/5 in its default configuration has a Medium Ballistic Hardpoint. A Medium Ballistic Hardpoint can mount any ballistic weapon that fits into 7 spaces.
Any configuration which has an AC/10, LB-10X, or AC/20 in its default configuration has a Large Ballistic Hardpoint. A Large Ballistic Hardpoint can mount any ballistic weapon.
Missile Weapons
Any configuration which has an SRM-2, Streak SRM-2, SRM-4 or LRM-5 in its default configuration has a Small Missile Hardpoint. A Small Missile Hardpoint can mount any missile weapon that fits into 2 spaces.
Any configuration which has an SRM-6, LRM-10, or Narc launcher in its default configuration has a Medium Missile Hardpoint. A Medium Missile Hardpoint can mount any missile weapon that fits into 4 spaces.
Any configuration which has an LRM-15 or LRM-20 in its default configuration has a Large Missile Hardpoint. A Large Missile Hardpoint can mount any missile weapon.
That way, you are never locked into "sidegrade"-only; you can always upgrade weapons to the next size bigger, but not two or more sizes bigger. I.e., a Machinegun could become an AC/2 but not an AC/5; a Medium Laser could become a Large Laser but not a PPC.
Note that several 'mechs have more hard points than weapons. This is always to allow "splitting" - for example, an Awesome 8Q has 3 PPCs, but each PPC location actually has two hard points. In such situations, the 'extra' hard points should always be one size *smaller* than the actual weapon - so the Awesome 8Q would have 3 Large Energy Hardpoints, and 3 Medium Energy Hardpoints, allowing it to boat 6 Large Lasers but only 3 PPC's. The Catapult A1 has an LRM-15 in each arm; thus, each arm has one Large Missile Hardpoint and two Medium Missile Hardpoints.
(Also note that this returns the Awesome-8Q to its original role as the only 'mech *designed* to PPC-boat.)
Edited by Ialdabaoth, 28 June 2013 - 07:17 PM.