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For us normal guys some might say a moot point, but for some in certain applications can be a big deal.
What is a Shock Wave of bullet flight?
Starting and staying subsonic.
When a bullet travels subsonic the shock wave ahead of it actually begins to flow out of the way dissipating before the bullet reaches it. The pressure waves that have been created, the bullet is always behind of it and the air stays at being smooth and clean.
Starting above but becoming subsonic
As a bullet that was initially fired above being subsonic becomes subsonic before target the shock waves catch up to bullet slapping it in the rear section (pressure waves), the bullet stability and accuracy is compromised.
Though with subsonic loads trajectory issues is another matter. Just kind of interesting to why one might use a .416 Barrett vs a 50 BMG being a sniper.(faster to target /no shockwaves)enhanced long distance.