The adverse effects of throwing a double 1 when shooting seem to be the same a throwing a single 1. Triple 1 is catostophic. Is this right?
I assume you are referring to the automatic weapons from the basic rule book?
In that case, Footnote 1 only applies to submachineguns, while Footnote 2 only applies to Light and Heavy machineguns (as indicated in the stat table).
The reasoning behind it is as follows:
Basically, submachineguns have smaller magazines, and, therefore, run out of ammo more quickly/easily.
The heavier ones, depicting early types of MGs prior to the 1930s, have more ammo (therefore the double 1 for out-of-ammo results), but might jam in a "terminal" (regarding the current game/battle) fashion.
It's also a bit of a risk factor to balance the high damage potential (both in killing and suppression effects) of the larger guns.