Obviously that's because one of my friend only gets "6" while he is beaming ... (so many times ... just wondering about his girlfriend schedule)
And of course he is playing ISA ...
He is the only guy I know who gets between 6 and 8 dice per beam shot whith only one at the beggining. Needless to say he rarely scores 1 & 2 criticals, often 4 or 6 which is ... nasty no ;-)
He IS the "infinite" power of a beam
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Concerning the "double" or "triple" or even "quad", I agree with
"the more dice you throw the more likely it is to score a critical"
But don't you like to "tempt the devil" , I'll throw 2 dice but if one gets through you're "quad" dead

I find this fun just to try to score a "triple" or so critical.
It is a matter of damage score versus the effects of criticals.
(i.e -1 speed 10 times is still -1 Speed and with a speed 10 or more you don't really care but if you have "only" 20 in damage (or less) you care about damage multipliers ... no no not thinking about a kind of WS ;-) )
Concerning the math part of the thing I don't think we should really care about the edges (even if it can happen from a math point of view).
Especially when dealing with beams.
Just keep in mind that one lucky hit can occur, that's life, but that's what gaming is all about. You can be the best general on earth (or minbar ;-) ) if you're opponent his lucky he his lucky and you are dead.
Can't fight against luck so keep close to stats and just think about
how many damage do I want to do ?
What's the proba to do it ?
Do I want to try it or do I choose another option ?
Given that you might want to incapacitate your ennemy as much as possible which will often come back to "I want to throw more and more dice"