It's about trying to break everyone through court costs and legal fees, not necessarily winning a judgement. They'll probably break the gunshop, dependent upon the distributor, they might break him as well. And, while they may not break Bushmaster, they can cause them financial harm through litigation win, lose, or draw. And, that is what they are after.
I'd also bet that the parents of the Newtown victims are not footing the bill for their Attorneys, if I had to guess Bloomberg is, directly or indirectly through one of his favorite anti-gun groups.