I use an A1 with 6x LRM5+art, BAP, CC, advanced sensor, and advanced zoom and just added LRM5 cooldown maxed out. Other hardware includes AMS, a 300 XL, double heat sinks and endo steel. It's unlocked to elite level as well.
I routinly get 4-5 kill games, with match scores in the 90's and 100's. The hit percentage is coming in at around 27% on my stats page, and would be higher but a LOT of missle blow into dead mechs. It takes me about 35-40 seconds to kill an assault mech (IS or clan) if everything is going well. I've one salvo'ed heavies coming straight at me as well.
It's basically the meanest, cruelest build possible. People bag on "lrm boats" and how it takes no skill. But playing fast and agressive, using the map and team members to your advantage it can blow people who think they know what they're doing out of the water. While HIGHLY dependant on teamwork, the 6x LRM5 build I have can carry pug games. Using the advanced zoom and firing on targets without locks is extremly effective as well. Sometimes I wonder if peopel know that's a possibility.
I've played around a bunch in the testing grounds, as well as seeing what happens in matches. The LRM5 benifits from being a smaller launcher so missles hit closer together. High arching fire means they come down on or close to a lot of cockpit hit boxes as well, or with moving mechs, tend to hit legs and back.
In getting to the elite level, I tried a lot of differnt builds. This is far and above the most sucessful. Leave it in chain fire, act like a missle machine gun, and watch mech crumble.
To put in persoective, many of my LOSING games in pug matches are 80-90k c-bill games. Winning matches have been 100-150K. Of course, playing only pug matches, I have games where I get killed in the first 30 seconds because no one turns around to see why I'm asking for help getting kiled by ECM lights. BUT, at least to me, the price of having the occasional letdown map is worth the epic murder fest the rest of the matches can be.
If you ever see a black catapult screaming around spitting a never ending stream of LRMs, say hi