I like the idea of jamming and what kind of ammo your using, heres what i have to say about all of it.
Jamming should be based on 3 things. Most importantly the quality of the weapon your using, then the skill level you have for using that type of weapon, and lastly the quality of ammo your using. (If you like to make some random stuff you can add in the amount of luck you have, towards a succesfull use of the weapon, but this should be miniscule).
Your weapons should have quality ratings, conditions like
Junk
You think its a weapon
Usefull
Deadly
Bringer of the dead!
(as you can see the ratings here go from worst to best)
and depending on the condition of the weapon is how well it works durring combat (relating to things like how much damage it does, how accuracte it is, and how often it jams)
All of these things can also relate to how good you are at using that tpye of weapon (for example if your using a pistol and have 150% on small weapons skill, even though its at "you think its a weapon" level, you can still do good damage with it every time)
Now with the ammo, conditions could be something like
Junk
It fires...
It does damage...
Kills every time!
Bringer of satan!
or something like that (as you can see i'm not too good at coming up with funny stuff, ehhh... i tried).
Now the quality of the ammo depends on where you get it, if you buy it from BoS, it will be "Kills every time" and if you steal it from an abondoned and radiated place (like the glow) it will be "Bringer of Satan" THis is becuase its an old military facility.. and its radiated so nobody can loot it (ok, ghouls could, but they are too lazy) so you found the jackpot... in the expense that the ammo is heavier than normal, and you had to get radiated to get it.
stuff like that, please tell me what you think