The 3L was my first mech (thank you, Daniel Rutter) and I have run all the popular variants from the streaks to the srm6 to the ERLL sniper. I've never been able to settle on a build that felt quite right. Missiles feel too imprecise; in my experience big damage numbers mean that not enough of your shots contributed to the kill. Sniping, while it can be effective, is just too boring for my playstyle (I really ought to be a Jenner pilot, but I can't bring myself to pilot a 35 ton stubby erection on fat legs), and I always felt like the underdog in a fight with another fast light. My solution was a shorter range build that simplifies weapons into a single group so I can spend more time thinking about piloting at high speed. I'm not going to argue the suboptimal nature of the LPLas in most areas as it is hardly debatable, but it works very well with standard MLas range profile and cycle time; replacing it with a LLas I found my cycle times becoming increasingly staggered which required me to spend more time on target and soak more fire. I much prefer the "Get in, alpha, get out" playstyle facilitated by this combination. The extra 2.6 alpha over the sniper build can make engagements a little shorter as well, which can be the few seconds that keep you alive. TL;DR - Look, it only costs you the price of a LPLas to try it out.
