The only game I've experiened gun jamming mechanics in has been Gears of War. For the most part, I loved it! Nothing says "awwwww fuck..." like having your gun jammed as you sprint out into battle to lay down some heavy fire onto dug-in grubs. The reactons are priceless and generally involve some type of 180 and dash for cover while Marcus or Dom bash their weapons like gorillas and scream nasty insults at them. But to say that the game is realistic in how they treat their weapons is going out on a limb (I mean, curb stomp for the Longshot is going to be a full out sledgehammer blow with the butt while holding the barrel in GoW2) with all that gooey chainsaw bayonette fun they have.
However! the mechanic was implemented in such a way that you never felt cheated for having the gun jam. Because of the slider, it was entirely your fault for having Marcus fiddle with his gun while masculinity pussed from him to coterize wounds from the hailstorm of bullets aimed at his face and crotch (beautiful imagery, no?). After hearing everything above my comment, I think this was probably the best way to go about it, in my opinion.
As for how they could do it in the future? I love the sliding bar, but I also do love realism. Something that was implemented in GoW was that depending on what gun you were realoding, the perfect reload space and the lesser "you didn't fuck up completely, good job" space varied in size. Why not make it similar to that? If you decide to crawl through some mud, maybe those bars get a little smaller in size so that there's more chance to jam for a moment. Maybe if you get shot a lot or find yourself near explosions often, then your gun realod bar should be effected. And maybe, JUST MAYBE, if you decide to use your sniper rifle as a baseball bat to beat around your opponent, then shove the barrel through his eyes and fire to make a foutain of greymatter and gore, that could possibly effect your jamming bar as well

.
In any case, I'm all for gun jamming if it's done properly and it makes it my fault for not doing something that can prevent it. I can't see myself playing a game where your gun will jam "no matter what at some point cause it's random chance lolololol!!1!!1!eleven!1!"