Hello! I was under the impression that any weapon eligible to fire during a fire-action (=a unit firing a tracer) would need to pass a reliability test to see whether it would actually fire. Or perhaps the % indicated by the reliability stat is simply substracted from the number of weapons allowed to fire. But this is - of course - just my theory. There is no clear information available..
This is how I suppose a fire-action works: During a fire action, a unit fires off one of the weapon-types that are available to it. It's pretty easy to tell which weapon was fired by taking a close look at the ammo-stocks of the unit* and by looking at the colour of the tracer (yellow = small arms, grey = bombard/indirect, red = any kind of ammo fired by big (AT) guns and all kinds of AT-weapons). Also, I suspect that a unit does NOT fire all weapons of a certain class (e.g. all small arms weapons: MGs, rifles, etc.), but rather just a single weapon-type (only one type of MG fires, but not the rifles). This is no big deal though if you keep in mind that units may very well carry out a fire-action every second minute or so (of course depending on it's cohesion, current level of suppression, etc.). I suppose that the amount weapons (of the given type) that are allowed to fire in a single fire-action are determined first by the firing unit's formation and the position of the enemy (front/rear/flank?) and then, my guess is that all eligible weapons have to pass their reliability test or you substract the amount indicated by the reliability stat to come up with the total number of weapons allowed to fire. Then all other factors would be taken into consideration (cohesion, training, cover, etc.) in order to determine the actual effectiveness of the fire.
* Note that ammo for standard small arm weapons (karabiners, rifles) seems to get spent even if the unit does not carry out any fire-action (= fires a tracer). Just being engaged (blue lamp) is enough for standard small arms weapons' ammo getting spent. I'm not sure whether this slow trickle of ammo has any effect on the enemy and/or whether these standard small arms weapons actually ever lead to a real firing-action (lead to a yellow tracer being fired) or whether they're just ignored.