I incline towards the one-test-per-weapon answer. The main debate in my mind stems from the fact that 5th edition has a different wording than 4th edition. Fourth edition, page 32, stated:
"When the firing of a single enemy unit inflicts casualties with pinning weapons, the target must take a Leadership test to avoid being pinned" (emphasis added).
Fifth edition, page 31, says:
If a unit other than a vehicle suffers any unsaved wounds from a pinning weapon, it must immediately take a Pinning test. This is a normal Leadership test.
If the unit fails the test, it is immediately forced to go to ground (as described on page 24). As the unit has already taken its saves, going to ground does not protect it against the fire of the pinning weapon that caused the test (or indeed of any other weapon fired by the same unit that phase) - it's too late!
As long as the tests are passed, a unit may be called upon to take multiple Pinning tests in a single turn, but if a unit has already gone to ground, no further Pinning tests are taken.
The main question, to my mind, is this: what causes Pinning tests?
In 4th edition, the answer to that question was "the firing of a single enemy unit" which "inflict[ed] casualties with pinning weapons."
In 5th edition, I believe the answer to that question is, "the fire of the pinning weapon" that inflicted "unsaved wounds" (emphasis added). The text does not read "the fire of the pinning weapon(s)" or "the fire of the unit." [EDIT: I would probably agree with Old Paladin if that were all the rule said. But it specifically picks out "the pinning weapon" that caused the test, which causes me to read the rule as stating that the cause of a Pinning test should be traced back to a particular individual weapon.]
The single-test position, as I understand it, advocates one of the following two positions:
- "Pinning weapons do not cause Pinning tests; the shooting of a unit equipped with pinning weapons, one or more of which caused an unsaved wound, causes Pinning tests."
- "All wounds from a single unit's pinning weapons are resolved simultaneously, and multiple tests of any kind can only be taken if the triggering events do not occur simultaneously, therefore even though each successfully wounding pinning weapon in a unit causes a Pinning test, only one test need be taken."
I disagree with each of these positions for the following reasons.
The first seems to me like a case of saying that the fifth edition language, despite being different from the fourth edition language, means the same thing. I do not find that persuasive. "The firing of a single enemy unit" does not, in my mind, mean the same thing as "the fire of the pinning weapon" or "a pinning weapon."
The second seems to me a case of making up a rule (the idea that simultaneous tests of the same type need only be rolled for once) that is not found anywhere in the rulebook. There exist in the game situations which might feel similar, such as the Morale test for taking 25% casualties in a single non-Assault phase, but that rule specifies that the test is taken at the end of the phase. The pinning rule, on the other hand, specifies that the test is taken "immediately," and specifies that multiple tests may be taken per turn unless the unit has gone to ground (either voluntarily or from pinning).
There seems to be a school of thought that thinks that "simultaneously" somehow means "as a single event." This is simply not true. Simultaneous events happen at the same time, but that's all. They don't need to have the same result. This is true even if the events are identical. Consider, for instance, the effect of eight shots from four heavy bolters in a devastator squad hitting a trukk. All eight shots came from the same squad and are part of the same shooting attack, so they occur simultaneously. All eight shots have the same Strength and are opposed by the same Armor Value, so each hit is an identical event. Yet in this situation most players find it intuitively obvious that each of these simultaneous, identical events must be resolved individually.
I don't see the principle in the fifth edition rulebook that makes Pinning any different.