I just witnessed this for the first time with a m4 bushmaster. I noticed my A3 target does the same thing, whats the deal with this? Bad for the rifle? A design flaw/feature?

Thanks

It’s the law of physics working perfectly. Kind of like slapping the top of the ketchup bottle when it's upside down.  Won’t hurt a thing.

The carrier holds the release in the extended position, so when there is the slightest bit of pressure taken off the release, such as the carrier going rearward from slapping the stock (ketchup bottle), the bolt release will go into the recessed position letting the carrier go into battery.