In T20 Starship combat the to hit roll is generally a given, especially for larger factored weapons. The to hit proceedures are simply Take the weapon factor, add it to the gunnery skill, (modified by any appropiate feats) add it to the Computer model number, (If your sensor operator can make his roll, if not there is no penality.) plus your commander's Intel/Wisdom bonus (whichever is higher) or 1/3rd his leadership skill roll plus a D20. You roll this against the Targets Armor Class. Which can't be higher, at TL 15, than 32. And the only way to get 32 is a small craft, with an agility of 6 and 15 points of armor. A ship, between 10 KTons and 99.999 KTons can't, at TL-15, be higher than 29, Bigger ships can't be higher than 27. Now against a Meson weapon the Armor doesn't count but your Meson Screen does. So a ship with a factor 9 meson screen will be AC 26 (If you can fit a factor 9 meson screen into a fighter), 23 or 21. (Same size ranges.) (-6 to +5 depending on configuration.)
The average gunner, according to the THB, has an 11 point gunnery skill. So a factor 9 Meson Bay a gunnery skill of 11 and a Leader with a leadership skill of 10 means you are rolling 23+ a D20 (Possibly plus your Computer model number. If your squadron commander has Fleet Tactics and you are in communication with the flag ship, plus his wis/int bonus.)
So shooting at a dispersed structure 75KTon Cruiser with a Factor 9 Meson screen and an agility of 6, you would need to roll a 5 to hit on a D20. If you roll a 15 or better then it is a Critical Threat, you then reroll to hit and if you roll a regular hit again it is a Critical hit. This is important because the normal hit is going to do 9D20 of damage + 9D12 of Radiation damage. YOu subtract one dice for each level of meson screen, starting from the lowest die until you are left with one die, then any remaining screen value is subtracted from the score on the highest die. (In this case -1 from the highest die.) Further, reading the Rules, it says that a Nuclear damper reduces the Radiation damage. So you will roll 9 D12, take the highest one, subtract 10, assuming factor 9 Nuclear dampers, and that is how many points of Radiation damage you do. Plus 9D20, take the highest die and subtract 1 and that is how much structural damage you do. If either of those hits does damage you also roll on the internal damage tables. (Once for each.)
Now on a Crit, Armor is ignored for reducing damage. (And Meson Screens are considered Armor against a Meson hit.) Further damage is multiplied by the Crit Multiplier. (In the case of a Meson weapon the crit multiplier is x10) Or 9D20 x10. + 9D12 x 10. Though whether Radiation damage is multiplied and or counts against the structure of the ship is unclear in the THB.
Now Spinal mounts A is 24 +D20, B is 25 +D20 and C is 26+ D20. A Factor D Meson Spinal to hit roll is 27 +D20. But a 1 is always a miss. (And against any capital ship that means anything bigger than C hits on a roll of 2+ regardless of screen and configuration) Spinal Mesons do a flat 16D20 (+16D12 Radiation) damage and critical threat is 5 points less or 10+ on the die followed by a to hit roll of 2+ and the damage destroys everything under 500,000 tons without including the Radiation damage, and renders anything under 8,000,000 tons a drifting but repairable hulk. (Again that is ignoring the radiation damage.)
A Normal hit (Which happens 47.5% of the time) is going to do your highest 7D20 out of 16 dice damage plus your highest D12-2 out of 16D12 radiation damage. (With the same assumptions of Meson screen 9 Nuclear Dampers 9.)
That is a snapshot of how T20 amd Meson weapons works. Mounting multiple Spinals is definitely overkill. 6 Factor 9 Meson Bays is likely to vaporize anything you might want to shoot at and 3 factor 9 meson bays are likely to bring even a Tigress to less than 0 SI. 2 Spinal Mesons is likely to kill anything you might want to shoot at. (If one Crits and the other doesn't you still vaporize a Tigress ignoring your radiation damage.)
For starship design, T20 is equivalent to HG. If you design a ship in HG it basically directly ports to T20. Combat on the other hand is obviously different.
Originally posted by The Oz:
Meson bays also use a large number of energy points (speaking in CT/HG terms), which most smaller ships can't afford. But from what Bhoins said earlier, in T20 meson bays are the way to go.
I'm curious, in T20 is it much harder to hit (and penetrate) a target with a meson bay than it is using a spinal meson weapon? It should be, as Sigg said it should be very difficult for a meson bay to hit a high-Agility target, and it should be near impossible to penetrate even a weak meson screen without a serious computer advantage.