I never, or possibly very rarely, ever see the complete absence of blow by before bullet exit. I do assume the bullet seals the barrel, and a video where gas is escaping AS the bullets exits would point to some serious inaccuracy questions.
If we view a cartridge and its neck as little different than a plastic straw in how it behaves under pressure, then even though a solid object may fit tightly in the end of it, nominally, once highly pressurized, the straw will expand like a balloon taking the shape of its container, even, and especially if the bullet were not allowed to move at all - in which case the gas would expand the neck and leak out around the bullet.
If that were to happen, it will expand from the back end of the bullet toward the front, not all at once, and certainly not from the front of the neck to the back, just like those animal tube balloons inflate from the source of pressure, forward. As a matter of degree, I think this is always happening to a greater or lesser extent.
Of course the bullet is never totally held in place, and the question is whether it can be jammed into the leade more easily than the neck can be expanded. It’s easily imaginable that on “some neck with some jammed bullet”, the bullet could move simultaneously with the neck expansion like a zipper unzipping, under pressure, but this may be a coincidence, rather than opening and “releasing” the bullet being a “causal” happening.
With no tension and little barrel resistance, then a thick neck might keep its shape until peak pressure, when that neck and the whole case expands and snaps back. If it had not expanded yet, then it might expand from “front to back” (the bullet is gone) as peak pressure - by definition - pushes dirty gas backwards, staining the exterior of brass with unsealed necks 1/3 of an inch down. We know peak pressure is toward mid barrel, not in the case, and this is clearly apparent to us in that rifle brass is only hot to the touch if peak-pressure-temperature gas is redirected into contact with brass, in the ejection cycle.
If we view a cartridge and its neck as little different than a plastic straw in how it behaves under pressure, then even though a solid object may fit tightly in the end of it, nominally, once highly pressurized, the straw will expand like a balloon taking the shape of its container, even, and especially if the bullet were not allowed to move at all - in which case the gas would expand the neck and leak out around the bullet.
If that were to happen, it will expand from the back end of the bullet toward the front, not all at once, and certainly not from the front of the neck to the back, just like those animal tube balloons inflate from the source of pressure, forward. As a matter of degree, I think this is always happening to a greater or lesser extent.
Of course the bullet is never totally held in place, and the question is whether it can be jammed into the leade more easily than the neck can be expanded. It’s easily imaginable that on “some neck with some jammed bullet”, the bullet could move simultaneously with the neck expansion like a zipper unzipping, under pressure, but this may be a coincidence, rather than opening and “releasing” the bullet being a “causal” happening.
With no tension and little barrel resistance, then a thick neck might keep its shape until peak pressure, when that neck and the whole case expands and snaps back. If it had not expanded yet, then it might expand from “front to back” (the bullet is gone) as peak pressure - by definition - pushes dirty gas backwards, staining the exterior of brass with unsealed necks 1/3 of an inch down. We know peak pressure is toward mid barrel, not in the case, and this is clearly apparent to us in that rifle brass is only hot to the touch if peak-pressure-temperature gas is redirected into contact with brass, in the ejection cycle.
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-Al
