A few points, Clark, that you may recall from RCR Battleschool;
A section's task, ideally, will be to assault nothing larger than a single enemy or two, so it may take several soldiers firing on an individual for a truly 'win the firefight'=effective 'pin'.
A section reacts to 'effective' enemy fire, and is trained to NOT be pinned; to ignore or shake off the effects. That means that as dangerous as it might be, you have to return fire and win back the firefight if you've lost it. If you don't they WILL do what you intended to do to them...close with and destroy.
The only thing that will get in the way of this is a soldiers instinct to stay alive, which will be at odds with his training.
When Ross and I were developing our 'War Game' we had this debate; His point was always along the lines of: 'I will never allow my troops to -insert our term for being pinned- they will be better trained than that...why would they choose to lay down and wait to die?...we don't train to do that.'
My point was usually 'They don't always have a choice when their instincts kick in...sometimes they will get down and become one with mother earth, even if it's not tactically correct. We don't train to do it, but we train with the assumption that our enemies will do it.'
Another point was that winning the firefight is often about perception. An anti-tank rocket should certainly be able to pin. An M-72 fired at an enemy trench and hits close by, even if it doesn't kill is going to have a shock-effect and will likely result in keeping their heads down for a couple breaths. We have already established that suffering from a pin effect is not necessarily a logical reaction, it is a gut reaction.
Taking cover is logical. Getting yourself 'pinned' is not.
Another point is Initiative! How many times did I harp at young section-commanders-in-training that you have to 'seize and maintain the initiative!' It is probably the most vital aspect in a firefight, and fairly easy to model in the game.
In respect with the above, I suggest:
Every weapon be given a pin rating. If another weapon stat is truly undesired, then perhaps go with the weapon's ROF (Though in many cases it would be better to customize the weapon's capability to pin).
When you shoot at an enemy figure and 'miss' within the pin rating (if the pin rating is 2 and you need a roll of 5 to kill, a 3 or higher will pin.) The pin effect is simply making that figure 'fired'. He loses any unused movement and attacks.
When you kill an enemy figure...actually kill him, you will have an automatic pin effect on other figures around him. You can also have this effect if you wound a figure, if you are also using some kind of wound system)
So if you had a weapon with a pin stat of 2 then all figures within 2" of that figure automatically become 'fired'. They lose any movement and fire actions that haven't been taken yet. This is the 'pin' effect.
If the attackers keep winning initiative they can use their superior fire to keep the enemy from doing anything. You are not getting cheated out of kills if you have good, accurate weapons. However if you have lots of firepower heading in the general direction of an enemy, you will likely pin him, if not kill him outright.
If the pinned defenders manage to win initiative, then they have a chance of winning back the firefight, so having unused leadership and command points become vitally important to seizing and maintaining the initiative.
Another plus is that the pin effect is not a suggestion that a soldier can choose to ignore and keep fighting. It is, for that moment, something imperative. Strong leadership however can get troops to recover and potentially turn the tables.
Submitted for evisceration by the gallery.