I believe it is a combination of both the FPSC version updates (specific to the guns information) and the model pack not updated to support these weapon changes. I have Model Pack 4 and here is a test from my v1.17...
Prisoner using colt45
Prisoner using Thompson
As you can see with the second, the rifle butt goes through the right shoulder of the character. In the first, the character holds the colt45 but it is not held in a grip- more like "attached" to hand by velcro.
There are some characters from older packs that did not really hold weapons well, and although you can assign a weapon to them, it may have been in hindsight during the upgrades or character creation. Some packs were not updated as far as where the gun should be held because of the fact there are many versions of FPSC used and each version has different changes. Any change in v1.19 and up will show anomalies in lower versions (such as v1.16 or lower). In my case, I note which characters do use weapons properly and which do not so that I can avoid things like this. It will never be fixed for everyone, and not everyone will be running one version of FPSC.
There's no problem that can't be solved without applying a little scripting.
