Untrue. Multiple attempts does not ensure at least one success.
If you're only looking for
one success though, multiple attempts does increase the chance you'll succeed though.
It's something like:
1 - (Chance of failure)
AttemptsIf you're flipping a coin, the odds you'll get a heads result on following number of attempts is:
1 flip: 50%
2 flips: 75%
3 flips: 87.5%
4 lips: 93.75%
5 flips: 96.875%
6 flips: ~98.438%
7 flips: ~99.219%
Which applies to using one 'big' gun versus several small ones. If you're looking for just one 'success' (in this case, breaching the rear armour on a 'mech), the bigger the difference in number of hits needed, the more useful the big gun can be even if the smaller guns deal more damage overall.
For an in game example, let's say you're fighting a squad of Battle Armour that's covered in laser reflective armour. The measuring stick seems to be 2 ReMLs versus 5 MLs right? 12 damage versus 10. So let's keep things simple and say that the suits have 9 armour (plus 1 trouper inside) so that it requires 10 damage to kill one suit. Average wise, assuming a 100% hit rate every turn, both the 2 ReMLs and the 5 MLs will both kill one trouper per turn- or a total of four turns to wipe out the squad.
However, each trouper needs multiple 'successes' (individual hits) to kill him. There's a 100% probability that one of the four suits will be struck by the first laser- then there's a 25% chance of 'success' in hitting the same trooper for every weapon afterwards. The ReMLs need 1 success and the MLs need 4.
-Turn 1-ReML:
25% chance of killing a trouper outright. (1 Success)
75% chance of hitting two separate troupers (1 Failure)
-Turn 2-8.3% chance of killing a second trouper outright (1 success last turn, and then 1 success on the new trouper)
16.7% chance of killing a trouper on the first turn, and damaging two troupers on Turn 2. (1 success last turn, and 1 failure on the new trouper)
18.75% chance you didn't kill any troupers last turn, but kill both troupers hit this turn. (1 failure last turn, 2 success against both damaged troupers)
37.50% chance you didn't kill any troupers last turn, but kill one trouper this turn and damage the remaining two (1 failure last turn, 1 success this turn)
18.75% chance you have 4 troupers still alive, all with 4 armour points left. (3 failures across both turns)
It's been years since I took statistics then, and I don't have the time to plot out five medium lasers but the odds you'll hit the same trouper with all five lasers on the first turn is only 0.39%
It'll work out to around the same number of turns to kill the squad, but you'll cripple the troupers outright faster with the ReMLs, meaning they'll deal less damage to you over that period of time. I.e, there's a 25% chance the ReMLs will reduce the squad to 3 troupers on the first turn- meaning there's only 3 troupers firing upon you in the second turn instead of 4.
It's the same thing as taking, say, a Gauss Rifle versus an LRM 20 against a light 'mech. Odds are that they'll deal around the same amount of damage over time, but the Gauss Rifle will likely cripple the light 'mech faster (taking out a leg, or a side torso first while the LRMs will saturate the target).
Your point about only needing one shot to penetrate the armour is a good one. Assuming a 'mech has 5 armour points on it's rear side torsos and a to-hit of 8:
42% chance of hitting (15/36) and 28% chance of hitting either the Left or Right torso (10/36) means that you have a 22% chance you'll hit with at least one laser and get internals:
18% chance of hitting with both lasers then gives a 48% of hitting a side torso (~9%)
49% chance of hitting with one laser then gives the 28% chance of hitting a side torso (~13%)
33% chance of missing with both lasers (0%).
The odds that you'll hit 3 out of the 5 MLs and that all three hit the *same* torso?
I'd rather not do the math, but it's clearly a lot lower.