"Going to sleep" = stabilizing gyroscopically in flight.
The fight path of a bullet is not a 'straight line'. It actually follows a movement know as 'precession' where the bullet actually spirals around the intended flight path.
Due to number of variable which include but are in no way limited to barrel twist, muzzle velocity, type of rifling, construction of the bullet. powder burn rate, neck tension and other factors ad nauseum, the radius of this precession, i.e, the deviation from actual line of flight, and therefore intended point of impact, varies during bullet flight. it is very common for certain bullets, especially high aspect ratio bullets ( bullets that are longer by several multiples of their diameter, generally regarded as high ballistic coefficient, low drag, long range projectiles) to precess, or 'wobble' more when leaving the muzzle, leaving befuddled shooters with seemingly large groups at 100 yards, but as the gyroscopic forces on the bullet stabilize, the precession radius becomes smaller, and consequently groups at longer ranges may not only be smaller in terms of MOA or MILs, but actually in size as well compared to 100 yard groups. Berger VLDs, among other excellent low B.C. projectiles, are known to exhibit ths seemingly contradictory flight characteristic.
A very good comparison that has been used to explain this is a simple child's toy top. When first set to spinning by whatever mechanism, the top tends to wobble at first, then settles into a smooth, almost noiseless rotation stabilized by its gyroscopic nature. Then, as the top rpm drops, it begins to wander again, becoming very wobbly indeed until it falls over. This is not completely unlike what happens to a bullet as it passes through the transonic transition to subsonic flight, albeit the latter is exceedingly more complex.
In short, referring to a bullet "going to sleep" refers to the bullet attaining desirable gyroscopic stability in flight after leaving the rifle's barrel. This can occur in as little as 20 yards to as much as 200 yards or more, and makes the shape of a group, not its actual size, the primary indicator of a particular load's accuracy. A load that forms up a predictable round group at 100 yards is generally more desirable than a group that is smaller in size but irregular in shape (note, this is my experience only and will likely generate a great deal of discussion regarding my mother's choice of footwear, the likelihood that I was in fact dropped on my head during birth, and the effect of flourescent lighting in my developmental years). I hope this removes someof the murk surrounding this topic, and best of luck to you.
Wes
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