You're forgetting terrain damage multipliers. Things that can make a hole in the outer hull need to do 100+ terrain damage, which in UFO were only blaster bombs - explosives do 50% of their power as terrain damage by default, so it needs to be an explosive that does 200+ or has extra terrain damage.
Projectiles do a 25-75% terrain damage roll by default, so it meeds to be a bullet with 134+ power or extra terrain damage. If you're not to the plasma cutter/fusion torch, likely you don't have anything that will breach outer UFO hulls.