On the other hand, we've all had that insanely frustrating moment where our Spess Mehreens have failed on LD10 (thanks anyway, Librarian!) when forcibly disembarked, and end up sitting in the open waiting to die without making nay contribution to saving themselves.
Most of the time though, Pinning doesn't have any meaningful impact, to the point where taking a Pinning weapon is never worth it for it's own sake, but for other characteristics or simple cheapness.
This is inherently 'wrong', and so needs a radical revision.
This is how I think it should function:
Unless you are Fearless, having a Pinning Weapon successfully score a Wound on your unit, whether you make a save or not, causes your unit to be Pinned. A Pinned unit is considered to have Gone to Ground between the resolution of the current attacks and the start of their next turn - though it is important to note that you do not receive the Cover Save bonus against the unit that inflicted the Pinning.
At the start of their next turn, a Pinned unit may make a Leadership test in order to become Unpinned, and return to normal. For a Stubborn unit, this is rolled against the Ld value as normal, however under other circumstances the value is modified by the number of Unsaved Wounds inflicted upon the unit by Shooting while Pinned.* For example, a unit of Imperial Guardsmen (Ld8 with a Sergeant present) is Pinned by a Space Marine Whirlwind firing on the unit. Three Guardsmen are killed in the blast, meaning that their Ld value is reduced to 5 for their Unpinning Check. They roll a 6 and fail, immediately resuming their Pinned status.
Where a Pinned unit is Assaulted - or more accurately, when an enemy unit attempts to declare an Assault move into the unit - immediately take an Unpinning Check for the unit as above. If the test is failed, the fragile nerve of the troops has failed, and they immediately Fall Back as with a failed Morale Check. They are considered Broken as normal from this point forwards, following the normal rules for Morale (being Broken supersedes being Pinned) and of course with the possibility existing of being destroyed if they fail to Fall Back far enough and fail to Regroup when Assaulted. In circumstances where the Pinned unit does Fall Back far enough to evade Assault, the Assaulting unit may redirect and declare an Assault against a unit that was within 3" of the Pinned unit before it Fell Back (if no unit exists, the Assaulting Unit may instead make a normal Consolidation move.)
This deliberately creates circumstances wherein the possibility to force a unit to stay out of action is increased the more suppressive fire is directed at them - with Pinning weapons actually able to act in a suppressive fashion. Ordnance Weapons have an additional -1 to the Test, Barrage Weapons have an additional -1 modifier to the test, and Ordnance Barrage are even more terrifying than either, and thus they combine the two for a total of -2 before calculating Wound modifier. The Wound modifier itself is simple and easy to remember, being consistent with the mechanic for Ld tests in Close Combat.
Low Ld units are much more likely to stay down when shot at in this fashion, and since they usually will take more casualties too, this works out pretty well.
Monstrous Creatures need an inbuilt exception to this rule, but that'd be part and parcel of their significant rules revision, not part of the Pinning rules itself.
One important note is that this differs from Entanglement (the situation wherein you are forced out of an Exploded vehicle.)
In the case of an Entanglement, follow the procedure below;
- Place the models in the crater you have replaced the destroyed vehicle with on the table. All models must be at least partially within the confines of the crater if possible.
- Between the flames, debris, smoke, and the fact that they're picking themselves up and attempting to reorganise, the disembarked unit is considered to be Gone to Ground [Except that if they are Assaulted, they simply return to normal, instead of following the normal process or the new one above.]
- At the start of their next turn, take a Leadership test for the unit. This test is automatically failed if any models from the unit were placed wholly outside of the crater formed in the explosion.**
- If the test is passed, the unit may act as normal this turn.
- If the test is failed, the unit may not move this turn, save to position any models not within 2" of the crater edge closer to any edge of the crater and/or to regain Coherency. Obviously models may not be moved 'out of the way' to create firing opportunities for other models. The unit may shoot as normal, though they count as moving. Assault moves may be declared as normal (the proximity of enemy forces overriding the need to regroup and reorganise somewhat.)
- At the end of the Movement Phase, the unit is considered returned to normal, and is no longer Entangled.
* - I had initially considered only having Wounds inflicted by Pinning weapons matter, but the idea of truly suppressing a unit with massed firepower appeals more, I think. If unbalanced, it can always return to the original way in which I envisaged it.
** - Note there is no exception for Fearless units - you might don't afraid of anything, but you can still get trapped in burning wreckage, and spend vital time trying to find other members of your unit or weaponry/wargear separated from you by the explosion.