Was messing around in mechlab and came up with this build. Takes full advantage of the BJ-1X's quirks to make a mean alpha at respectable ranges (nearly 500 meters with range module with the LPlas!) Hot, but the LPlas quirks make it more manageable. Leg armor is low, feel free to drop engine/some MLas for more. You could also drop 1 Mlas for an extra heatsink, it has just enough slots for that
I feel the need to chime in and add my 2 cents for this build. It's the best killer I've ever run. I think it has to do with the LPL quirks, as well as the energy duration reduction in general. This build out of over 50 mechs has my highest KDR simply because it cores out enemies at ungodly speeds. It currently sets at 45/6, 7.5 KDR, and almost seems too easy to run. My first time running it I got over 1000 damage, with no basic skills. I make a few changes to benefit my play-style however. I find the quirks lower heat generation noticeably for both the LPLs and the MLs to the point where I remove all but 1 extra DHS, max the armor out expect for 16 in the head, and upgrade the engine to an XL295. It's worth the upgrade over the XL280, as it's only an extra ton. I also put all the lasers in the arms. Those high mounted weapons and quick tracking arms help immensely. I usually favor using the LPLs as much as possible, but when I want someone to die, I alpha all the weapons with one deadly pinpoint punch. That 42 damage hits just like a Wubmaster or Thunderbolt 5S because of the duration quirks. The best part? No one seems to expect it from a Blackjack. More damage than a BoomJager at longer ranges, smaller stature, all at 116kph. What's not to like?
8ML is what I ran before quirks and that already was my favourite medium mech. 2LPL+4ML is a nice upgrade, especially for standard pug drops., But I don't use the mech in CW anymore...it feels way too squishy for the range it offers, even with the opportunity to hide very well.
This is my take on this build. I really prefer to be able to put out some extra dmg with more Heatsinks. Low legarmor, yes, but I have yet to be legged in one of my BlackJacks.