Ok, I've got a half decent weapon tech tree set up now...
1) Researching Physics provides new warhead types; kinetic, chemical, nuclear, antimatter, polaron, quantum, ion, etc.
2) Researching Military Science provides new delivery mechanisms; projectile, pulse, beam, torpedo, missile.
Combine any two to produce a new weapon.
EG: Polaron + Beam = PPB
Kinetic + Projectile = DUC
Also, Chemical + Beam = Ripper beam
Nuclear + Beam = Incinerator Beam
Quantum + Beam = WMG
to reuse a few good names
Researching level one of both warhead and mechanism, will open the specific weapon branch.
1) The weapon branch provides cheaper weapons
2) Researching above level 1 in the warhead or delivery techs will increase firepower/range/accuracy/etc depending on what makes sense for the combo.