If the source is modulated and it's output is controlled into a good beam the receiver optics won't matter much, in fact you'll want as wide a viewing angle as possible, or you can 'lean' to avoid shots. If the gun transmits something like an address at the carrier frequency (who is shooting) and the senders lasers is well focused all you need to do is have the receiver require a certain number of packets (100ms's worth maybe) to get a 'good' shot. Ideally you'd want at least two receivers, front and back, maybe one on each shoulder that require a slightly longer reception pattern.
You can think of it this way. Unless you're a sniper and that does not really come into play here, you're going to shoot a target down with multiple rounds. Your receiver starts to beep the instant it detects another gun firing within a lethal pattern, giving you time to react or take cover, a few interrupts will reset the 'kill' counter. Too long in the kill zone and you get flagged with a death. Given the known acuracy/reliablity statistics you can find on modern fire arms and you could replicate the kill probability of any fire arm. Even if the user had no recoil and a scope =)