As it was explained to me by the good folks at Sierra, when the firing pin hits the primer, it drives the brass forward against the datum line of the chamber. As the powder builds pressure, the case grabs the chamber walls but there's not enough pressure to push the case back against the bolt. Obviously, we aren't talking about big measurements here. Because the case can't move back, there's nothing to stop the primer from backing up until it hits the bolt. Per them, it won't hurt the gun at all. I ran several hundred rounds thru my Model 14 before it went away.
Really wished I never sold that gun. It's the ideal kids hunting cartridge and gun. Small, compact, and same power as the 30-30. I've been looking for another one in 25 Remington for several years now.