If I'm not misinterpreting "run and gun" as playstyle (which defintely could be), then it's what I'm doing most of the time myself
The chassis which really suits me in this, is a lot of fun to play for me and I have the best results in is the
BlackJack.
(ok, haven't got many different chassis yet - so it's the best so far for me). I'm playing them all as close-quarter escorts / supporters / finishers / assassins - ah the heck, you pack so much firepower in these little mean machines, you'll blowing up things left and right.
I'm not so into missiles neither, and none of the BJ's sports missile hardpoints. (@Koniving: yeah ok, none of the
implemented BJ variants as of
yet 
), so you can skip those and go for energy & ballistic (or energy only, depends on the BJ variant).
Many say the BJ's are too fragile... but for me they're just working. I achieve better results (and survive longer) in them as in my Thunderbolts - go figure. Seems BJ's just suits my playstyle very well
So maybe you just want to give the BJ's a try.
(we can deepen this further if you like, why they work for me, talking about builds, acting on the field etc.)