Let me add a few thoughts...
In real life, to shoot at anything effectively beyond point blank range with any missile weapon inherently requires a LOT of aiming. When using bows in a military battlefield setting, the "target" is a large block of solders so its just shoot at the "crowd" rather than an individual which makes aiming a lot simpler and typically the shooter adjusts for wind and distance with each shot (kind of like using tracers). So HARP (and most RPG games) distort fire rates for missile weapons since its based on load and fire in a battlefield setting. In HARP, since rounds are only two seconds, aiming at a "new" or moving target SHOULD require "aiming" for one extra round per Range Increment (RI) beyond 2 - just to qualify for shooting at the target. Getting a target in your initial sights at a distance is not so easy. Correcting for wind and distance can take a number of seconds to "gauge". At any distance beyond point blank range, a called shot is essentially impossible. Ask a bow hunter at what range they like to take their shots - its generally under 25 meters.
The Careful Aim action introduced by Nicholas seems reasonable - although I'm not sure I would allow reducing the RI 8 penalty completely just because you aimed for 40 rounds... I think only allowing the penalty to be reduced by no more than half for all RI seems more reasonable.
With firearms, stabilizing the weapon by "bracing" or using a shooting tripod makes a huge difference. High quality gunsights (scopes) make an additional huge difference. Beta Blockers help as well (a heart medicine that reduces jitters and of course banned in competition shooting). Bracing and scopes allow BOTH faster and better aiming - so it should take less long with a scope to achieve "best possible aim".
Attempting to do less than lethal damage is already allowed in Base HARP with the Subdual action:
Subdual: At times, characters may wish to strike a foe
without utilizing their entire force for the blow, in an effort
to disable or subdue a target. This action receives a -20
modifier to the attack roll. Should the attack still result in
severe damaging effects, the character may adjust the result
to any other lesser critical on the same table at their leisure.
Another important point is this. IF the target is AWARE of the shooter, aiming might be more difficult because the target will use movement and cover to spoil the aim by turning their body, keeping their head and chest behind the shield, etc.
Let me illustrate an example...
I played semi-pro Paintball for 9+ years (I was a star player on a team that competed in the national tournaments), my paintgun including the scope and all the trimmings cost me a frightening sum close to $500. You can spend twice that or more. A cheap paintgun in the hands of an expert can hit a head sized target 9/10 times from about 10 meters. A high quality gun can thwack someone's head 4/5 times from about 20 meters. A cheap gun would be lucky to hit the target 1/10 times at 20 meters. So quality can make a huge difference in the effective range. I used a shorter barrel than most sacrificing long range accuracy for "maneuverability" in the bushes... The "sniper" setups in the hands of an expert could thwack people at ranges of 40 meters or more. Of course, in the woods its difficult to actually SEE anyone at 20 meters [those darn paintballs hurt - so players take pains not to be seen]. In effect, paintguns are about as accurate as a bow but much less effective at longer ranges - paintballs bounce instead of break once they lose velocity.
Anyway, at close range you can literally twist your body to avoid being shot by watching the other player's aim and trigger pull. More than a few times, I've blundered into an ambush, dodged the "aimed" shot and returned fire (commonly called "snap shooting") and marked (hit) the ambushers without being marked myself to the ambushers' everlasting annoyance. Before I played Paintball - I would have said you couldn't do that. But you can. It's a skill I and others practiced. Remaining calm and cool under that pressure is critical as well. Most players cannot. When moving under missile fire, serpentining and "juking/jiving" is also critical to spoiling the aim of enemy fire - unseen or no. It really makes a difference.