Looks like this can occur with motion blur off aswell, so likely it's a TAA motion vector issue.
Similar issue occurs in UE5 (see [Link Removed])
Download the licensee repro here - https://github.com/erebel55/RenderBug
Boot in 4.27, PIE, move around the level, the gun with blur quite intensely.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-116769 in the post.