The headspace in a blow-back .22 rim-fire is the distance from the breech block face to the forward rim cutout in the barrel.
Like any other rimmed round, the .22 rim-fire case stops its forward movement on the rim cutout in the barrel.
Spring pressure normally holds the breech block in contact with the head of the case, but the spring does not play any real part in the blow-back portion of the cycle.
The spring is at maximum extension at that point and does not provide enough force to slow things down once the cartridge fires (and if it did you would not be able to pull the block back to chamber a round).
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