Great question, let's hope there are some equally great answers to follow.
Gunshots come in many different shapes and sizes (single shot, automatic, small caliber, large caliber, suppressed, silenced), as well as different environments (interior, exterior, small spaces, large spaces) and flavors (antique, modern, and futuristic weaponry). Each one has its own characteristic, and by the way, there are no rules. :) That's the good news.
You have the basic breakdown correct: Attack, body, punch and mechanic. (There are more, arguably, but I think those four cover the food groups pretty well). So, knowing the pieces, you just have to start arranging them to make your particular puzzle complete.
Not knowing what kind of gunshot you are trying to design, it's difficult to go into much detail about what will/won't work. What I do recommend is to put away your plugins and put on your listening ears. If you have a shooting range nearby, get a day pass and learn what real guns sound like (better yet, do some shooting yourself - personal experience can be an extremely valuable education, regardless of the subject matter). If you have a film/TV show/game that you are trying to emulate, play those scenes over and over and dissect what makes the guns sound so good.
If I had to recommend some plugins, I would say that a compressor/limiter is a useful tool to increase the apparant loudness of the shot (Waves MV2, L2, etc), while a tranient enhancer of some kind will restore the bite or attack (Waves TransX, SPL, etc). For low end enhancement you can go to Waves RBass or MaxxBass (I promise I'm not a sales rep for Waves :)).
Lastly, just sit down and start experimenting with different elements. Slight timing adjustments can make a HUGE difference, as can short/long reverbs and delays.