WTF GUYS. SRSLY?!
If you had 100% multishot, it'd be 10% + 10%*90% thus 19% of having at least one critical (thus it includes every combination of 0+1, 1+0 and 1+1)
BUT
Since you can only have 90% multishot on rifles (which is what the OP asked) the probabilities might be a little bit less, something around 18.1% as some have suggested above, though I'm not so sure it'd be that precise. Anyway it'd be something between 18 and 19% for sure.
Thank you all so much, I appreciate the help. If you wanted to see what exactly this was for go to: My Scratch program. Thanks!
TL:DR Version
Multishot doesn't seems to affect crit chance. Weapon crit chance is probably what the stat sheet says.
Long Version
This discussion leads me to believe that the description of multishot is misleading.
"Multishot describes the number of pellets (bullets, projectiles, bolts, etc.) the weapon will fire per attack. While multishot is a feature normally encountered mainly on shotguns, it can be added to any weapon through mods, effectively allowing you to fire more bullets without consuming extra ammunition."
The way I understand this, is that when you fire your weapon that instance of your shot has crit chance calculated... Meaning that regardless of how many projectiles come out of your gun the crit chance is calculated only once. Ex. You fire a sobek, all the pellets that land either crit or do not. There is no instance of some critting and some not critting.
Multishot doesn't seems to affect crit chance. Weapon crit chance is probably what the stat sheet says.
Long Version
This discussion leads me to believe that the description of multishot is misleading.
"Multishot describes the number of pellets (bullets, projectiles, bolts, etc.) the weapon will fire per attack. While multishot is a feature normally encountered mainly on shotguns, it can be added to any weapon through mods, effectively allowing you to fire more bullets without consuming extra ammunition."
The way I understand this, is that when you fire your weapon that instance of your shot has crit chance calculated... Meaning that regardless of how many projectiles come out of your gun the crit chance is calculated only once. Ex. You fire a sobek, all the pellets that land either crit or do not. There is no instance of some critting and some not critting.
..No
The person above me. Technically speaking sort of. It'll increase the chance of you getting a crit in any given scenario. However your over all critical hits, if a large enough sample is recorded, won't be changed. I.e. you fire 1000 bullets, multishot included, and you fire 1000 bullets without multishot you'll achieve approximately the same number of crits. You'll just get there faster.
Suuuuure, they either all crit or none of them do ... https://i.imgur.com/ZisYRRK.png
To get back to the necroed topic: Multishot doesn't increase Critical Chance per se, it only increases the number of shots, but more shots mean you have more chances to see some yellow numbers, it's not technically the same as increasing critical chance though.