Well, the spring does a little more than nothing.
A normal buffer that is in-battery has weights inside that could be resting at any point within the free-travel area of the interior of the buffer. This means that it is possible that, during firing, the bolt could begin to unlock and ONLY encounter the mass of the buffer body, which is aluminum (weighs ~.4oz) for the first 1/4" of travel of the bolt carrier group.
This means that during the critical unlocking phase, the bolt and carrier could accelerate to the rear MUCH too quickly and unlock too soon since virtually zero resistance is offered by the buffer. The additional problem here is that, in a normal buffer this effect is unpredictable.
The "silencing spring" in the A5 buffer is not an accident and is not there purely to quiet the mechanism, though it certainly has that effect. It is also there so that the full mass of the buffer assembly is presented to the rear of the bolt carrier while it is in battery, thereby offering CONSISTENT resistance during the unlocking phase of the bolt carrier group and guaranteeing that the BCG doesn't see only the mass of the buffer body (which might as well be just air).
I have also seen this have the effect of stabilizing the cyclic rate when shooting strings of full auto.
It's science.

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