So, I went and tested every sniper rifle, shooting at each and every dummy in the firing range in the 9 (head), 7 (neck), 8 (torso), and 5 (lower leg).
First thing I wanted to do is to apologize: the explanation I gave for how sniper rifles damage work is entirely wrong. Sure, they were explanations I got from supposedly serious YouTube videos and the charts they produced, but still, I should not have used such arrogance and definitive tone for something I had never checked in details myself.
To be clear however, you are not quite right yourself: there is simply no multiplier whatsoever.
At short range, ALL sniper rifles deal the same damage: 100 to the head, 80 to the sweet spot, 64 elsewhere. This despite the fact they don't have the same base damage (hence the absence of any multiplier, these are just set figures).
Over the range, the damage will vary from gun to gun, which makes sense as you enter the sweet spot range for some of them. For those that are long range (PSR and L115), the damage remains 100/80/64 all the way to the 125m mark. For those that have a sweet spot before 125m, the damage increases until the sweet spot and decreases afterwards.
The mini scout is the exception here: its damage decreases with distance, starting at the same 100/80/64 at the closest target. Impossible to say how low it goes from the range itself.
Which does lead me to my second remark: I had completely forgotten most sniper rifles had a base damage of 100. After all, when sniping, for me it's either headshot or a miss, I never really cared for the actual numbers. But that leads to a big question:
You want sniper rifles to deal max damage at close range no matter the body part. For every sniper rifle but the mini scout, that would be a one-shot kill even if shooting a finger. Can I assume you didn't mean that, and mostly based your experience and frustration on the mini scout stats?