It is true that sectoids can be taken down by pistol fire fairly easily
but now you have given me more crazy stuff to put down!
He = health, AF = armour front, AS = armour side, AR = armour rear and AU = armour under
Alien Race/Rank He AF AS AR AU
Floater Soldier 35 8 6 4 12
Floater Navigator 35 8 6 4 12
Floater Medic 35 8 6 4 12
Floater Engineer 35 8 6 4 12
Floater Leader 40 16 12 8 12
Floater Commander 45 24 18 12 16
Sectoid Soldier 30 4 3 2 2
Sectoid Navigator 30 4 3 2 2
Sectoid Medic 30 4 3 2 2
Sectoid Engineer 30 4 3 2 2
Sectoid Leader 30 4 3 2 2
Sectoid Commander 30 4 3 2 2
Snakeman Soldier 45 20 18 16 12
Snakeman Navigator 45 20 18 16 12
Snakeman Engineer 45 20 18 16 12
Snakeman Leader 45 20 24 22 20
Snakeman Commander 55 26 26 22 20
Ethereal Soldier 55 35 35 35 35
Ethereal Leader 55 40 40 40 40
Ethereal Commander 55 45 45 45 45
Muton Soldier 125 20 20 20 10
Muton Navigator 125 24 24 24 15
Muton Engineer 125 28 28 28 20
Reaper 148 28 28 28 4
Chryssalid 96 34 34 34 34
Silacoid 114 50 50 50 10
Celatid 68 20 20 20 20
Sectopod 96 145 130 100 90
Cyberdisc 120 34 34 34 34
Zombie 84 4 4 4 4
Heavy Cannon does 56 AP, Rifle does 30AP, Pistol does 26AP.
Cyberdiscs are 20% resistant to AP, mutons and zombies are 40% resistant to AP
Weapons can do 0% to 200% of their listed damage but for purposes of test let's just assume they do listed damage.
All sectoids are the same regardless of rank, 30 health and 4 front armour. This means to kill them in one shot you need to do 34 damage so rifle and pistol can't guarantee a one-hit kill, where a heavy cannon can (although technically nothing in XCOM can guarantee 1-hit kill as all weapons can do 0 damage), however you can fire more shots with rifles and pistols and 2 shots will take down any sectoid.
Floaters are a bit more tough, 35-40 health with armour of 8-24 meaning you need to do 43-64 damage to do a one hit kill depending on floater rank so heavy cannons are far more likely to take out any floater except for a commander in one hit when compared to rifles and pistols. Pistols in-fact will only barely scratch a floater commander but seeing as they will only be in alien bases and battleships it's unlikely you will be using standard human weapons vs these alien scum. Floater leaders however are a more common threat. With 16 armour it means rifles will do 14 damage, pistols 10 and heavy cannon will do 40 damage, given that floater leaders have 40 health it means you will need 4 pistol shots, 3 rifle shots or 1 heavy cannon shot so vs that foe a heavy cannon might be a better choice.
Snakemen have even more health and armour so obviously heavy cannon is much better choice but snakemen are generally seen later in game when you are likely to have lasers and lasers > heavy cannon.
Cyberdiscs though are a real and present danger while only armed with puny human weapons. Cyberdiscs have 120 health and 34 armour (on all sides!) so they require a big punch to do any damage, in fact as rifles and pistols only do 30 and 26 damage respectively they are likely to just bounce off a cyberdisc's armour harmlessly. A heavy cannon will pack a punch though to at least do a bit of damage, after armour they should be doing 22 damage thus taking around 6 shots to take down a cyberdisc with a heavy cannon. However lest we forget cyberdiscs are resistant to human bullets! That 56 damage a heavy cannon does will actually be doing ~45 damage instead and when taken after cyberdisc armour it means doing damage of 11 per shot meaning you need 11 shots to take down the floating heavy weapons platform.
So in conclusion if you want to kill aliens in 1 shot then heavy cannon is your human weapon man, if you want to kill aliens in one turn then rifles and pistols are fine, if your facing cyberdiscs though then it's probably better to just pack rockets, a cyberdisc would probably need 1 rocket and 4 heavy cannon shots to kill, or 1 large rocket and 2 heavy cannon shots. When you consider reaction fire though sometimes it's better to be take that 1-shot kill instead of needing 2 or 3 which means that although rifles and pistols are fit for purpose vs sectoids and floaters, a heavy cannon can let a soldier live longer, although that's assuming they are strong enough to carry the heavy cannon and ammo in the first place otherwise the TU reduction can make them like turtles who don't react very fast
This is math :> if you shoot 2 times with 50% on average you will hit once (aka 100% :>).
This have simple equation:
probability * number of trials = number of hits on avg
Its because two extremes (2hits and 0hits) have same probability and they nullify each other.
If you shoot 2 times with 50% chance then the chance that you hit at least once is 75%