This is a post to patch up some rules regarding warheads and special effects.
see Common Fighters of the Third Imperium by Mark Gellis has the rules for the missiles.
Warhead's size and Power. The bigger the size the more power is assumed. Size really matters because a 10mmx40mm Case Less stingray round, SM-9 (B550), will not override a SM+9 Ship. What we can do is make an assumption that the attack modifier is -10 vs HT modified by the size difference.
UT weapons and size modifiers. This is purely the weapon. if used as a war-head you have to account for the body. Use B550, Use UT's ammo weight to get mass of each ammo.
Pistol ammo 10mm x 25mm (SM-11)
Pistol Ammo 15mm x 37.5mm (SM-10)
10mm x40mm (SM-10)
15mm x60mm (SM-9)
18.5mm x70mm (SM-9)
25mm x100mm (SM-6)
40mm x160mm (SM-6)
64mm x256mm (SM-5); See UT145 for missile body cost, x10 for high endurance guided missile body which ha 1.25 minutes of powered flight.
100mm x400mm (SM-4) 20lbs warhead; $300 from size modifier and UT145 missile body. x10 per guided high endurance missile body 3.75 minutes of powered flight. (note: reduce cost, weight by 8 factor from ISW226 stats)
200mm x800mm (SM-3) 160lbs warhead (missile body is 1.13 yards for a total of 2yrd of SM+0)
(see 500mm missile)
25mm stingray (SM-6) vs SM+0 Power armor. HT-10 reduced by the size difference: SM-6 vs SM+0 is 6; reduce the HT-10 by 6 to HT-4)
64mm stingray (SM-5) vs SM+4 vehicle. HT-10 reduced by the size difference: SM-5 vs SM+4 is 9; reduce the HT-10 by 9 = HT-1 roll)
200mm stingray (SM-3) vs SM+5 Tank. HT-10 reduced by the size difference: SM-3 vs SM+5 is 8; reduce the HT-10 by 8 to HT-2.
Example Ship to Ship combat
200mm stingray (SM-3) vs SM+9 Ship. HT-10 reduced by the size difference: SM-3 vs SM+9 is 12; reduce the HT-10 by 12 to HT+2).
Note on EMP vs Stingray missiles UT156. Stingray missile use the Lethal electrical damage (B432) causing stun for minutes unlike EMP which cause stun for seconds but has an area effect AND has another -8 penalty (so HT-18 plus size difference).