Who defines "reasonable?" You think you alone do? In reality, once Hill and Carroll testify that they recognize this pistol with the identifying marks they put on it as the one seized from the defendant at the Texas Theatre," it's going to be considered authenticated at trial, whether you like it or not.
You think you alone know what would happen at a hypothetical trial?
The problem is that neither Hill nor Carroll "seized" anything from anybody, Carroll didn't know whose hand he grabbed a gun from, and he didn't mark anything until long after a gun left his possession.
No chain of custody whatsoever.